If the instructions below are confusing, it probably means you would benefit from lessons with a qualified riding instructor. Best of luck and remember, it is better to do simple things well than to ride advanced maneuvers in poor form. Ride in an absolutely straight line at a walk, trot and canter. Leg yield at the walk, trot, and canter. Move forward promptly from halt to walk, walk to trot, and halt to trot.
Half halt or check.
If you are able to perform all of these exercises with your horse, he should have not problem taking the correct lead every time. If you cannot perform these exercises, you need to work with an instructor that can help you develop the basics with your horse. First perfect the trot to canter transition, then the walk to canter transition.