Think of your circle as a diamond shape and every time you reach a point in your diamond, bend to your next point and shave off your corners. The correct bending of your horse will allow the shape of the circle to appear circular when you are going from point to point and shaving off the corners. Also don’t wait when you get to the point to bend your horse, continually bend because you are not on a straight line but one that curves.
Obstacles to stop them falling out or in. I recommend poles and a mounting block as an extra. That’s what I use
I put 4 poles in an X shape totalling 20m diameter. Make a Gap in the middle, so then you can ride straight centre lines too. Also in the quarter part, you can ride a smaller 10 meter circle
Think of your circle like a clock - look to 3 o clock, then 6, then 9, then 12
Cones can really help, and make sure you plan it and imagine exactly where your going to ride x
Make sure to use a lot of bend and practice it! You can’t expect it to be perfect at first. Practice it every time you ride and don’t give your horse mixed signals or it’ll make your horse confused and your job harder.