The hands should be near the front of the pommels. Chest and hips open. Heels back slightly under the horse. Repetition is the key. The mushroom has become an integral part of pommel horse training.
Most gyms have far more mushrooms than pommels horses.
Numbers and focus on technique are the keys to circles. Lots of tight stretched circles on a floor dome. Do more circles. Be sure that you are extended, don't get lazy. Don't start with your hips too high.