Teaching your horse this exercise is one of those things that makes perfect sense when you know how to do it, and I have taught this to numerous people, but always in person. It is very hard to explain on paper, especially on a horse new to the exercise. Your best option is to find a local dressage trainer and tell him or her that you want to learn how to teach your horse to do the stretch-down trot circles, but look for an instructor who teaches French Dressage, not German Dressage. If they don’t know what you are talking about, and can’t explain the difference between French Dressage and German Dressage, keep looking for a French Dressage trainer. I can tell you how to do it, but unless somebody who understands how to do this correct can be standing there to teach it to you either on your horse or on a horse that has already been trained how to do this, and correct mistakes as you work your way through to understanding what to do and why it works, you are likely to end up confused and with a horse that has no idea what you are asking him to do. For example this requires that you be able to remove the tension from a lot of muscles and ride relaxed, so that you can learn to feel what the horse beneath you is doing.