I normally start with a working pirouette, which is normally done on a 10-15m circle. I teach the horse to bring it's haunches into the circle while keeping a constant canter. It is also important to have some inner bending through the head and neck. For my feeling, I like to have the sensation that the horses shoulders are on the outer part of the circle and the haunches and bending on the inner part of the circle. It is also important for the rider to stay seated in the middle of their saddle. Once this is executed correctly, you can start making the circle smaller. You just have to make sure that your horse is keeping the rhythm in the canter.
Thanks for your question,