To sit your canter, remember to let your belly button go before your shoulders. Basically, you have to get the feel of it, then remember that and it's a lot easier to get time and time again. Also, be sure that your legs aren't stiff, or this will cause you to not be able to go with the flow of the canter. Allow your hips to follow the flow.
To get a slower canter, I would suggest asking with less leg and more seat, and also making sure you have enough connection. Wiggling his nose left and right could also help.
To ask for a canter from the walk is the same from the trot, inside leg at the girth, outside leg a hair back, use your seat. Basically, you have to make sure you have enough energy to do it. It takes a long time to be able to do it, but you have to practice. Whenever I am cantering from the walk on a horse that doesn't get it, I ask for it, and if he trots instead, I wait for a few steps and if he doesn't canter after 3 steps of trot I ask him to halt, then walk again. I continue it until he understands.
To stop from the canter, (this could be different with your horse, but I hope it helps) make sure you have connection in your outside rein. Ask to stop with your outside rein, and push your horse into the stop with your inside leg. When you're doing this, you have to make sure your horse knows that you don't want him to move over or go faster, just to stop. It may not work on your horse, it just depends on how he's been trained.
Hope these helped! (:
Eventing: the only sport where you wear your medical information strapped to your arm.