First you should teach your OTTB horse to walk, trot, and canter quietly in both directions. After some time spent on the training, a horse should be able to take both leads quietly. However, it can happen that your horse will be associating the canter with gallop for a long time. If that happens, try to focus on teaching him a walk-to-canter transition. Normally this kind of training calms down a horse and helps with teaching him how to canter properly.  At first, the horse should walk and canter in the same direction. As you progress in the training, you will be able to start asking him to canter in the opposite direction. When teaching your horse to canter, keep your hands relaxed and steady. If you feel like your horse is not ready, you can try to use your legs to guide him. But try not to move them too much or he might get confused.