didnt you post the same thing yesterday? Do you have a trainer/instructor? If so get them to help you - its easier to explain to someone face to face, and im sure your trainer would be able to help you - then he/she could advise as to what you are doing wrong/right at the same time.
If you are wanting to do this for show jumping purposes.
Put a pole at the end of the diagonal and practice over that, just try to sit still and reverse your aids at the pole.
If it is for dressage then you need to practice transitions on a straight line into canter,then practice counter canter and work on simple changes. Once the aids are fully understood you can ask for a change but do not lean over and try to practice your feel for the timing as the horse can only change during the moment of suspension and the canter will need to be of very good quality.
Years ago when I was competing I had a problem. My trainer got on and had no problem.Reason he improved the quality of the canter and the changes were easy with the amount of lift he got in the paces.