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.
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.