Set up two ground poles, with 4 strides in between. 4 fairly large steps equals 1 stride.
Canter through them and practice getting 4 strides. If your horse keeps getting 5, you need to lengthen his stride. If he gets 3 or 3 1/2 you need to steady. But all that is practice.
Try not to canter in blindly and adjust at the last second. Pick up your canter on how you think it should be, and canter that rhythm and pace all the way through. If you mess up, so what. It's information for the next time around. Staying steady and rhythmical is key for you and your horse to find the distances.