Sight-reading, that is, being able to play a piece of music without having seen, heard, or studied it before, is a valuable skill for any musician to have. Singing at sight in particular is a wonderful tool even for instrumentalists, as it hones the musician's aural skills and sense of pitch. Not to mention that for singers, that's just what sight-reading is. The trouble with this skill, however, is the pedagogy. How do you teach someone to be able to look at music and just sing it, with no prior experience with that piece of music? Music teachers everywhere struggle with it, and their students continue to lack sight-singing proficiency. Here I'll attempt to give some insight as to how to go about helping someone learn to sight-sing (including yourself!), based on how I've been taught and how I've tutored others. Please note that this is in reference to tonal pieces of music.
In short, teaching this skill follows along these general guidelines:
1. Familiarization with solfege.
2. Interval training and pitch memory exercises, and the use of guide tones.
3. Application of solfege syllables to music, with familiar and unfamiliar songs, first written and then at sight.
4. Introduction of altered syllables and minor mode (consider both do- and la-based minor: see below).
5. Mixed modes and modulation.
6. Sight-singing by interval alone (see below for details).
It seems to me that everyone hates this stuff, but if you don't have perfect pitch (and most of us don't), it's an absolutely fantastic tool for sight-singing. Unlike singing on numbers (1, 2, 3, 4, 5, 6, 7), you can account for accidentals (it's a bit of a hassle to sing "flat seven" on one note - far easier to have a single syllable like "te"). Numbers can also get confused with rhythms, should you have students speak them or write them in. In my opinion, solfege also has a leg up against note names, because using note names is akin to fixed do (explained momentarily), counting on the singer to be extremely proficient in theory, and many who struggle with sight-singing are not very experienced in music theory yet.
So here's an explanation of solfege.
Each syllable corresponds to a different scale degree of the major scale.
You've probably heard of these before (Sound of Music, anyone?). The ones you're likely less familiar with are the accidentals, i.e. the raised and lowered versions of these pitches:
Raised 3=(use fa)
(7 is already raised)
Lowered 1=(use ti)
Lowered 4=(use mi)
You'll notice that the system isn't perfect - there isn't a name for every altered scale degree. Then again, it's extremely unusual to see a flat tonic (1) or subdominant (4), or a sharp mediant (3). Usually these only occur around a modulation (key change) and can be avoided (covered later).
All of these syllables, including the altered ones, have corresponding hand signs. These are not necessary, but I personally believe they greatly enhance the effectiveness of solfege. I was taught hand signs when I was in elementary school and found that if I signed the syllables as I sight-read, I would sing the correct note even if I was unsure of myself just by making the hand sign for it. My brain knew what pitch the sign was, even when I hadn't consciously thought of it yet. I have friends who claim to have similar experiences - but those who added signs later and didn't learn them at the same time they learned the syllables don't seem to have had such a phenomenon. Hand signs are also useful for teaching music by ear: you can sign a melody and students "sight-read" your hands. This is good practice for them, too.
There are multiple ways to use solfege. For example, fixed vs moveable do: originally, do was "fixed" to always be C. Therefore a C major scale would be do-re-mi-fa-sol-la-ti-do, but D major would be re-mi-fi-sol-la-ti-di-re. Using fixed do tends to help students develop relative pitch, and innately does a good job of teaching key signatures, but it has a MUCH sharper learning curve (in terms of application, that is) than "moveable" do. With moveable do, do changes to be the tonic of the key. Therefore, in C major we have do-re-mi-fa-sol-la-ti-do where do is C, and in D major, we still have do-re-mi-fa-sol-la-ti-do, but do is D. Moveable do is more commonly used and has practical applications in teaching music theory (i.e. relationships between major and minor modes, modulations to closely related keys, etc).
Another choice to be made is whether to use do- or la-based minor. Each has a logical place, although for whatever reason many teachers only teach one or the other. In the case of minor, it depends whether the piece is natural, harmonic, or melodic, as well as whether it flirts with its parallel major or relative major. Do-based minor is based on the parallel minor, altering the third (and sometimes sixth and seventh). Natural do-based minor would be do-re-me-fa-sol-le-te-do, although it is more commonly used with harmonic and melodic minor. La-based minor is based on the relative minor, with la becoming tonic instead of do, so that a natural minor scale would be la-ti-do-re-mi-fa-sol-la. In the case of harmonic and melodic, fa and sol can be raised to fi and si, respectively. It makes sense to use both la- and do-based minor in different scenarios and personally I think it's worthwhile to teach both. Whichever you choose, however, make sure your students are comfortable with it AND understand the music theory behind it before you introduce the other. You're going to hear a lot of groaning about why they have to learn a new one, so be prepared to explain the logic behind your choice, and remind them that this kind of versatility will do wonders for their sight-singing once they're proficient.
(Steps 1 and 2 as described in overview)
The first step to teaching sight-singing with solfege is, of course, to learn solfege. Start by singing a major scale, do to do, with the syllables. When students can do so fluidly and without looking at them (it can be helpful to write them out for singers who haven't heard of them before), move on to exercises in intervals. For example, alternating thirds (do mi re fa mi sol fa la etc). It's important to be familiar with the syllables even out of order. Thirds are a good place to start, but also try fourths, fifths, sixths, even sevenths. The idea is to familiarize the student with intervals that they will find in music before they even see the music. This way, when presented with it, they can recognize jumps they may not have been able to sing before, using solfege as a guide for recognition.
(Step 3 as described in overview)
After singing with solfege comfortably for a while, it's important to actually apply it to music and the written page. For beginning students, start with a simple piece (C, G, and F major are always comfortable places to start) and have students write in the solfege. Prior to this, make sure you've established the key with them (i.e. 1 sharp means G major. G major means G will be do.) - don't leave them completely in the dark. Give them plenty of time at first, as they will probably need to count lines and spaces for many of the pitches. Next, sing the piece with solfege.
Continue having students write in their solfege, and slowly take it away. For example, have them only write in the solfege where there are leaps in the music, or more difficult passages. It's important that you eventually STOP letting students write in solfege altogether. With more advanced students, you can skip writing in the solfege and just jump straight into it. Any level of student, however, can make good practice by translating well-known songs (traditional music like Christmas carols, the national anthem, familiar hymns, or music already learned) to solfege. This not only hones aural skills in general (realizing the solfege is the first step to dictation/transcription), but demonstrates the practical application of solfege. And it's just dang good practice.
It's important to note that age and level are not the same in music. For example, a high schooler learning to read music might struggle more than a ten-year-old who has been reading music for several years already. In a classroom setting, you'll have lots of different levels of students, and the hardest part about teaching solfege and sight-reading will likely be finding a pace that won't frustrate your beginners too much or bore your more advanced students too much. Both are likely to happen at some point, of course, but I've noticed that most students tend to dislike sight-singing and solfege, so it's important to keep a good pace (with both challenges and simpler exercises to reassure them of their success) to avoid turning them off to it altogether.
Something to consider beyond using interval exercises to get the sounds in your students' heads:
Sight-singing depends on a certain level of pitch memory. Any note can be found if you can just remember where tonic is. It's a great exercise to stop students mid-reading and ask them to sing do, and then continue. Other good notes to do this with are sol, and then mi (and la, if you generally use la-based minor, as many choral directors do). These are "guide tones" and can be used in difficult passages to find other notes. For example, if students have trouble singing the sixth from do up to la, have them sing do-sol-la. Then ask them to sing do, pause and ask them to audiate sol (i.e. hear it in their heads, but don't sing it!), and then sing la. Finally, have them just sing la. Skip the second step for advanced students who just happened to stumble. This is also helpful for leaps around fa and ti, since they have that half-step pull to mi and do, respectively. Have students sing the resolution and then the troublesome note, then skip the added resolution and go straight to fa/ti, just like in the previous example. These methods help students to read music by relying on the guide tones that they hear in their own heads, rather than what the piano is playing for them. And it's far more comfortable to try to find a stray note from something familiar nearby (ex: fi to sol - use sol to find fi) than to try to pick it out of thin air. After time, however, you'll notice that this becomes unnecessary and students will be able to pick notes out of the air - because they've developed aural skills and every note of the major scale has essentially become its own guide tone. You'll know without a doubt that students are ready for sight-reading with tonicizations and modal mixture when they rarely need to use guide tones any more.
At this point, your students should be comfortable singing pieces in the major mode using solfege at sight, without writing it in first. They should also be able to sing a number of different interval drills at a good tempo (in college, that means quarter note at 120, doing the exercises in eighth-notes. You're the judge of what a reasonable speed for your students is, dependent on their experience and ability) and well-tuned. Poor pitch accuracy (in an able singer with otherwise reliable pitch) even when the syllable is right is a sign that it's time to slow down or back up. Also be aware of students singing the correct syllable but the wrong note (and vice versa, for that matter). They'll often notice it themselves, since after a certain level of familiarity with solfege, you can somehow just "feel" that your syllable doesn't match your pitch, but the point of solfege is to associate a pitch with a syllable so it's important to be consistent in matching them up.
So assuming your students are proficient, the next place to go is minor. See the solfege section about whether to use do- or la-based minor. In late middle school/early high school choir when we reached this point, my teachers started with natural minor (and la-based minor), and then harmonic, adding "si." On the other hand, in my aural skills class in college, we jumped straight to do-based harmonic minor, to create a connection between major and minor scales and precede modal mixture. I personally believe the second way is more practical in the long run, but it really depends how far you want to take things. I think it's wise to teach do-based minor as it maintains do as tonic, but la-based minor definitely has its place. Make your judgment based on repertoire you're teaching or your goals for your students. Teach both if you can.
Introduce minor first with scales and then a bit of music. When students have a feel for it, translate your old interval exercises into minor. This is tough! Make sure you consider all three types of minor (especially harmonic because it's so common and unique).
This is also a good time to spice things up in major. For example, pieces that briefly tonicize the dominant or sub-dominant (altered tones fi and te, respectively) can be easily introduced now. Use guide tones just like you did before (fi wants to go to sol, te wants to go to la).
At this point you can start expanding on the tonicizations of 4 and 5 to include other closely related keys (2, 3, 4, 5, and 6 in major, and 3, 4, 5, 6, and 7 in minor) with their leading tones. To move towards learning to sing a modulation, try a melodic passage that tonicizes with a leading tone (fi sol), and pause on the new temporary tonic (sol). Have students sing it as do. Try a scale or sing a cadence (I-V7-I, suggested: do-mi-sol-mi-do, ti-re-fa-sol-fa-re-ti, do-mi-sol-mi-do) to establish the key (nothing too extensive - this is a tonicization, not a modulation) and then (this is important!) have them find the old do. Have them study the music and discuss what changes the key to and from the tonicized pitch. Definitely sing it. Harmonize at the piano to firmly establish it. Being able to sight-sing music that changes key will rely on the singer's ability to visually recognize and then internally hear relationships between keys. Spending time discovering the relationship between closely related keys helps to develop this skill.
In pieces that actually change key and stay, do will change. In short excerpts like are often used for sight-singing, this may not come up, but in actual music, it often does. It's at your and the students' discretion whether to change do in short examples or to sing with altered syllables. I can't say which is better; it depends on the music. In longer music, have students change do. For example, if there is a long passage in the dominant, change sol to do. The exact point at which you do this also depends on the music. If the new do repeats (for example, in C major modulating to G major, two successive G's), sing the first as sol and the second as do, and keep it as such from there on out. In switching back, if you see the figure "do te" (pretty common) sing it as sol fa. Really this is all dependent on the music though, so encourage students to use their best judgment. Discuss with the class where good places to switch would be and why (i.e. te is a note from the original key, a good sign that you're headed back). Also realize that in sight-singing examples used in contests and auditions, changing do is RARELY going to be necessary. This is a tool that is practical for sight-reading music with the intention of learning it, and need not be something you focus on. Discuss it when it comes up and not if it doesn't (it will eventually anyways).
I would recommend focusing on tonicization before modal mixture, if only because working with tonicization continues to strengthen the guide tones, which will quite possibly become relied on again in passages that dance around parallel modes.
When students are comfortable in both major and minor, it's time to take it a step further and mix them together. A good way to do this is by singing chord progressions, preferably in whichever inversion leaves the bottom note as close to tonic as possible (i.e. I-IV-V would be do-mi-sol-mi-do, do-fa-la-fa-do, ti-re-sol-re-ti). Start with diatonic progressions in one mode and then switch between modes (for example adding diminished ii, bVI, and minor iv into major). For a simpler start, like with younger students, try I-V-i-V-I, or precede a chord from the other mode with its counterpart in its same mode (i.e. in major, sing IV before iv). Before sight-singing in mixed modes, write out the progression and have the students sing it. It'll sneak it into their ears and give them a little boost for their first shot.
Don't forget that even though your students are learning about tonicization and modal mixture, your sight-reading practice doesn't have to be (and probably shouldn't be) limited to examples containing this material. Completely diatonic examples with no altered pitches can still be difficult, and it's always important to throw in music where the rhythm is more challenging than the pitches. Remember to continue to present students with a variety of sight-singing examples: both those that correspond to the current lesson and those that expound on what they are already capable of.
I have to be honest with you: I'm still learning this myself, and so I can't give as detailed a description as the other parts of this write-up. I hope that the little insight that I do have to offer is helpful in spite of my lack of experience.
Sight-reading music that lacks a secure tonal center should be saved for students who are extremely proficient with everything mentioned above. Not to say that less experienced singers should not be introduced to it, but up until now, everything has been based on solfege (more specifically, on remembering tonic and other guide tones like the tonic triad, regardless of and in relationship to other pitches), and moving away from that before a student is quite secure in it could very easily diminish its effectiveness. Additionally, a certain amount of proficiency in music theory (recognition of triadic/chord outlines in a melody and knowledge of intervals, for example) is necessary for learning to sight-sing non-tonal music.
Sight-reading atonal music forces the musician to rely on intervals. It is possible to use solfege in spite of the lack of a tonal center, or in cases of very week or constantly changing tonal centers. However, it's easier to rely instead on the relationships between notes outside of key. Students just beginning to read this type of music can still rely on solfege in a different way, to hear intervals. First students need to be familiar with what intervals they've been singing, so go back to your interval exercises and name them (i.e. in the thirds, identify which are major and minor. Same for sixths; also consider perfect/augmented/diminished fourths and fifths, and half/whole steps/major/minor seconds.). Establish the easiest solfege combination for each interval (generally do to something else, although I personally find minor thirds easier to remember as la-do/do-la than do-me/me-do - you'll find that you have such preferences as well, and encourage students to use whichever solfege combination they hear most easily - sight-singing is an independent endeavor, after all). Start very slowly. Identify each interval, and assign an equivalent solfege interval. For example, you can use do-re for an ascending major second. Sing the excerpt using your solfege. You'll have to sing each note twice in order to change syllables. Be aware of triads - no need to sing do-me, do-mi when you could sing do-me-sol - and it's important that students begin to recognize such patterns. Continue by removing solfege.
Exercises to help reading such music: pick any note and have the choir sing an interval you choose for them. Make sure you specify ascending or descending (lots of students have more trouble going down than going up). Give students music and have them identify every interval (including quality) in a certain amount of time. You hardly have any time at all to think of what an interval is when you're sight-singing so it's important to be able to recognize them quickly/instantly.
A Final Note
You've probably noticed that much of learning to sight-sing involves developing aural skills, and that a certain level of music theory naturally accompanies those skills. I can't say it's necessary to teach music theory in order to teach sight-singing, but it does have a great impact on the effectiveness of using solfege (since solfege inherently reveals some function). It's certainly not a bad idea to pair your sight-singing lessons with tidbits of music theory.
I also should note that while having a piano play along with singers is common and can be helpful, sight-reading will suffer for it. If your goal is to produce great readers, let your accompanist rest while your singers read their music. On the other hand, having the pianist play harmonies (rather than the singers' parts) can be a good tool - having an idea of the harmony that fits with a melody can make it easier to audiate and can be helpful to reading. Also, it's perfectly alright to go to the piano to correct pitch issues that the singers can't resolve by themselves. Just don't jump the gun on it - give them a chance to hear it and fix it themselves first. It's very common for singers to rely too heavily on the piano.
Finally, know that this takes a LOT of time. If you stick to your knitting, you can learn all this in under 2 years. It requires daily practice (even if some days you only have time to sing an interval drill or two and a couple harmonic progressions). More than likely, these skills will be taught in a choral setting and therefore be spread over many more years than that. Try not to get frustrated (although most of us do, sooner or later, like with anything else that takes so long to develop).
Some resources for sight-singing I've found helpful:
Music for Sight-Singing (Ottman)
Passage to Music Literacy (Cho & Kteily-O'Sullivan)
Melodia (Cole & Lewis) <-- This one's been around since 1909 and is still very widely used!