The canter is a three-beat gait that is very, very easy to ride. It is a bit like a rocking horse, or sex. I was told this in no uncertain terms a lady screaming across the arena at a 15 year old me "ride him like you're fucking a mate." Apparently she has very skewed outlook on what fifteen year olds do in their spare time. Riding a canter the first few times can be a little daunting because of the speed, but once you calm down it's fairly simple.
When you are riding a canter you generally pretend like the first and main leg in the canter is a foreleg. This is to keep you remembering which leg should be leading, which makes it alright, but really, this is wrong. The first and main leg in the canter is the outside hind leg. It starts off on the ground while the other three legs are in the air, and it pushes the horse forward.
The next legs that come to touch the ground are a diagonal pair, the inside hind leg and the outside foreleg. This is very important as it is the inside hind leg that carries the horse in the canter. This means that the inside hind leg must come right up under the belly of the horse, otherwise it will not be able to correctly carry the weight of the body. The outside hind is lifting off as this happens, and the inside foreleg has not hit the ground yet.
The third step is when the inside foreleg touches the ground. Then diagonal pair (inside hind and outside fore) are lifted from the ground, and only the inside foreleg is on the ground. Barely a moment later, the inside foreleg is lifted from the ground and the horse is in the air.
This all happens very fast and makes the horse rock backwards and forwards gently, the wind brushes through the mane and it feels as though you are flying.
The leading leg is basically the same idea as the diagonal is in the trot.
Very simply, the leading leg is the inside foreleg. The way to remember this is to imagine that leg as being the center to the circle, or the maypole, to what you are doing. If the horse was to spin suddenly in the direction you are going you would not spin on the outside leg, you would spin on the inside leg, because it is easier. The leading leg is what the horse bends around.
To get the correct lead I need to explain how you ask a horse to canter, which I would have described anyway.
1. Are you in the correct position already?
This should entail heels down, shoulders up, looking ahead at where you want to go. Hands should be clasped around the reins and the elbows willing to bend, as the head moves much more in the canter than the trot, and you need to be ready to give this. However, before you canter you probably need to shorten the reins by a good four inches. Start sitting to the trot, nudge your horse on just a little bit and relax down into the saddle so you can feel the horse and he can feel your weight.
2. Be ready for coordination to the max!
Maybe I lied a little when I said that canter is very easy, I have been doing it for years and forgot that this part takes extreme concentration and a lot of practice. So, organize your legs. Your outside leg (the one closest to the wall, outside of arena or bend that you doing) should go back just behind the girth of the horse. Your inside leg (the other one) needs to go slightly forward towards the inside foreleg to remind it that it needs to start doing some work. At the same time you should move your inside hip forwards and your outside hip backwards so that you are swiveled slightly in the saddle. Turn the horse's head slightly to the inside, but don't make that rein tighter or heavier.
Why do this? The outside hind leg needs telling to move, since it is what will start the canter pace. The inside leg is not what carries the horse, but for all purposes we pretend that it is: it needs reminding also. The seat swivel is important to emphasize this, and the extra weight on the inside that the horse feels from your sitting slightly forward with your inside hip tells it which leg it must lead with, that is, the inside one, as it will have more weight to carry there and so will be encouraged to step up with its inside hind.
3. Now kick that bastard!
Shoulders up, legs pressing in but do not clench the reins. The horse will probably have realized what you are doing by this stage and will be raring to go, and you will feel as though you need to crouch down into the saddle and hold on tight with your reins. This is very bad. Many horses love cantering and will let out a little buck as you go into it. If your shoulders are not up and you are not sitting deeply in the saddle you will be unseated and you will fall off. Squeeze with your legs by giving more pressure on the inside than on the outside.
Sometimes horses don't like working! I never liked the long dressage whips for this, I preferred the short riding crops. Hold it in your inside hand. If the horse refused to canter instantly put both reins in your outside hand and crack the horse on the shoulder with the whip. I believe the tradition is hitting them on the flank just behind your leg, but I have had bad experiences with horses bucking from that, and a shoulder-whack will get the message across just the same.
I've heard a lot of different ways used to describe how to ride a canter, from pretending that your hips are an ice cream scoop and you are trying to scoop out ice cream from the saddle, to the idea that there is money under you and you cannot lift your butt from the saddle otherwise it will fly away. I've also been told that it is a lot like sex. Pretend it is Brad Pitt and don't lift your body from that saddle. This is easy and the horse basically tells you how to ride it, unlike the trot where you come away sick and bruised.
Number one: Do not raise your body from the saddle. You will feel as though you have to, but just don't. Not until you know your way around horses and you are doing something even more complicated like jumping.
Number two: rock your body through your hips, but not your shoulders. In the canter your body should be a bit floppier than in the trot, but you should not turn into a sack of nothingness. Your position should remain the same, but your hips should follow the movement the horse gives you to follow. This is a rolling movement, a thrusting movement or a "dear God I'm about to fall off" movement.
Some horses have very short canter strides so this rolling movement shall feel like no work at all. Others have a massive stride and leave you feeling as though your hips are broken. The longer strides are, however, infinitely more fun to ride that the short strides.
Steps 1 - 3 should happen in no more than 15 meters. I suggest that you come into a square corner (one corner, a couple meters of straightness, and then another corner) in which you can get yourself organized. In the first corner you sit trot and organize your legs. In the straight you ask the horse to trot a little faster, you shorten the reins and then in the middle of the next corner you really emphasize the turn you are doing and ask the horse to canter. Doing it in a corner will pretty much guarantee that you will get the correct leading leg.
Once you know what you are doing you don't need to start in a corner. Just by using your legs and seat to tell the horse which leading leg you want, you can do this all on a perfectly straight line, or even on a line bending in the opposite direction. When you ask the horse to use the incorrect leading leg, this is called counter canter and is a dressage move. It demonstrates how balanced the horse is.
A flying change happens when you want the horse to change leading legs. Flying changes require a lot of balance in the rider's seat and through the horse's body. Flying changes are not recommended for beginning riders or young or weak horses. To do a flying change, you change the position of your legs and seat so that the opposite leg is slightly forward, when compared to how they were when you started cantering. You generally need a nice big kick to ask a horse to do this. The actual flying change may feel (and look) a bit like a buck. If you are unsure about doing this, don't do it. If you are on the incorrect lead, bring the horse to a trot and start again.
Cantering from the walk is essentially the same as from a trot. You will need a very forward walk, something that feels as though it will spill over into a trot in the very next stride. To do this you can talk to your horse in an upbeat voice and swing your legs against his belly: as a horse walks his ribcage will swing from side to side and by using your legs to gently push it back you can make the horse walk faster. You can also rock your hips as though you are peddling backwards on a bike to get a horse to walk faster. Get a whip, change your seat and legs around, and get ready to let out a war-cry as you boot your horse forward. (Warning: the horse may buck, rear or otherwise misbehave with this activity. Sit deep in the saddle, shoulders up, and kick him again if he doesn't fall to the ground and start running. Don't be afraid. The best way to get a horse out of a rear is to kick him. If it is a buck, drag the reins up, kick and hold on.)
Riding a canter is physically exhausting for both you and the horse. It builds stomach muscles and tests muscles you never knew existed. Don't canter for too long or too often. If you canter lots the horse may get very excited and become difficult to handle.