Right then, those canter transitions. I confess to often messing up the first few of the day, and many riders do not ride these well, largely because they don't believe the transition will happen, so they over-do their aids.
A common strategy is to 'kick start' the horse into canter by leaning back and shoving with your backside. This means that you end up in waterski position as you 'push' the horse into 'motorboat'. He might just trot faster, or he might canter, but either way you have put your centre of gravity behind his, and will pay the price for this as he races off with you. The more you lean back, the more he speeds off. This is like a rug being pulled out from under your feet. It would slide away from you ever faster as you leant back.
Think of trot like the smooth surface of the ocean. The first canter stride is like a tsunami wave that arises out of it, swelling up and coming back down ready for the next wave or stride. Your aim is to go up with the wave, and come down with the wave, staying vertical throughout. If you fall off the back of the wave (either on the way up or at the top), you are unlikely to get canter, and if you do, you may well loose it after one stride. Many riders who lean back and 'kick start' the horse fall off the back of the wave. A smaller proportion of riders get 'in front of' the wave, and staying on exactly the right balance point is no mean achievement.
Realise that each successive canter stride is like another tsunami wave that you have to stay with.
The canter aid comes from the inside seat bone as much or more than it comes from the lower legs. The front inside shoulder of the horse lifts into canter, almost like taking off for a jump, and your inside seat bone lifts with it. In fact, lifting this seat bone can cue the horse to lift his shoulder, as if he were stuck to your underside. To make this work you have to be well anchored over the outside seat bone. (If all your weight is on the inside one, you can't move it!)
If you have trouble doing this, and/or you have trouble getting the right lead, think of looking over your outside shoulder. This is good advice at any time, as it makes your torso face to the outside, putting your inside hip and shoulder ahead of your outside hip and shoulder. This goes with 'inside leg on the girth, outside leg behind the girth', which you have probably heard
Canter is the four legged equivalent of skipping, and you (like the horse) could skip on either lead. If he has both inside legs in advance of his outside legs you must match him, by putting your inside seat bone and torso ahead of of your outside seat bone and torso.
This also means that facing your body to the inside (and reversing your seat bone position) would invite the wrong lead - especially if you also pulled on the the inside rein and made the horse 'jack-knife'. He would then fall on his outside shoulder as this is the only one that free to move. Result: you are on the wrong lead.
Think of keeping your outside rein and giving your inside hand during the transition. This is a good policy because it helps you not to pull, and not to face to the inside. The horse's inside shoulder is free, and the horse who feels that the hand brake is off will make a much neater job of the transition than the one who feels constrained.
At the same time, the more you can demote the transition inside your head and make it no big deal, the more likely it is to go well. Keep aiming to keep your body lined up, and make this a higher priority than getting canter at any cost.