Let's take a random line of Latin poetry.
impulerit. Tantæne animis cælestibus iræ?
Knowing that it's in dactylic hexameter, we know a few absolute rules about this line:
  1. The line will have six feet.
  2. Each foot will be either a dactyl (-^^) or a spondee (--).
  3. The last two feet will be a dactyl and a spondee (-^^|--).
  4. The very first syllable will be long (-), no matter what the first foot is.
Here's what we have now:
|-                            |-  ^ ^  | - - |
|impulerit. tantæne animis cæl|estibus | iræ?|
Right out of the gate, we need to look for elisions. Elision is the process of "leaving out" the last sound of a word and combining it with the first sound of the next. You elide two words if:
  • the first word ends in a vowel or the letter m AND
  • the next word begins with a vowel or the letter h.
Do we have any elisions in this line? Yes indeedy: "tantæne" and "animis" are going to elide. Here's how the line looks now:
|-                              |-  ^ ^  | - - |
|impulerit. tantæn(e) animis cæl|estibus | iræ?|
How do the remaining syllables scan? Let's find out.

A syllable is long if:
  • it's a diphthong (like æ or oe);
  • it's a vowel followed by two consonants (even if the second consonant is in a different word);
  • it's long by nature (the i in pueri, the i in hi).
Let's see what syllables we can mark long:
|-      -    -  -        | -   - |-  ^ ^  | - - |
|impulerit. tantæn(e) ani|mis cæl|estibus | iræ?|
From here on in, the line scans itself. Everything that we haven't marked as long is short. Final product:
|-  ^ ^|-    -| -     ^ ^| -   - |-  ^ ^  | - - |
|impulerit. tantæn(e) ani|mis cæl|estibus | iræ?|
Tune in next week for Elegaic couplets.