Essentially to get the inside leg to outside rein you "bend" the horse around your inside leg - using the inside hip pushed forward, toe pointing straight ahead and the outside rein prevents them from popping the shoulder. Don't forget that the horse follows your weight - so to circle left - left hip is forward, left rein slightly shorter than right (left elbow back further than right elbow and to inside of withers about and inche or so), right rein VERY slightly forward (unless it is a very small circle).
Then adjust your weight for horse falling inside or outside of circle and use legs (inside leg at girth, outside leg MAY need to be very slightly behind girth). Mostly the outside leg "block" or prevent the horse from throwing their butt to the outside of the cirlce, rather than applying pressure. (Think of applying pressure behind girth as asking for canter versus keeping outside leg slugfhtly behind girth with light contact on horses side but NO pressure unless they move their butt into your leg - then they should stop moving their butt when they feel more pressure against your leg.)
Hard to explain without being there. Hope this helps.
Dressage is for Trainers!