Sorry I didn't explain the leg yield well enough. Here is a link to a video that explains it so well. Leg Yield - Horse Training - YouTube
This guy explains it so well. Baby steps at the beginning but if you do what he suggests I think you will have a much easier time of riding.
Transitions- if you are trotting and want to transition down to a walk, put a leg on and as you come down to the walk, move the horse over a step of two.
Watch the video and teach the horse to leg yield looking at the fence. Don't leg yield in transitions until you master the steps in th video.
Quarterline- there is a centerline down a riding ring. The quarter lines are the lines between centerline and the fence. (On the long sides) sorry if this isn't clear. Turn your horse onto the quarter line and push him over to the rail.
But this video will show you better than I can explain it.