Start with simple changes with a few strides of trot and then canter again. Then reduce to one stride. Then introduce 9ver a pole
What I did to tech my gelding was I placed a pole in the ground and cantered over it in a circle, and once he had a steady canter I half halted him to the other rein and put my leg back (as if asking for a normal canter) and they should then be able to do it. Once they are fine with the pole, you can remove it and they should grasp the concept quickly. Otherwise if they don’t, just carry on with the ground pole!
Teaching the horse to do them or simply asking for one? If you want to ask you have to collect the canter, accentuate the helps, do a half halt and change the helps, your body weight can help you in the beginning