I teach all of my horses to back figure 8's for several reasons, the first obvious...you're going to have to back a circle or semi-circle at some point in your showmanship career, so let's perfect it and it also teaches the horse's that my subtle body cues can straighten out a crooked back up when I'm trying to actually back a straight line in a pattern in the show pen.
I usually use 2 cones just to give myself something to back around and I start with my horse in the center of those two cones and I back equal circles so that my eight looks like two circles OO and not an X that is rounded (X) on the ends.
What you need to remember when backing circles is that your horse MUST NOT PIVOT! It should take equal steps all the way around the cones.
Circle to the left (your horse will be closer to the cone than you), you need to remember to pull YOUR BODY away from the horse's head so that your horse has ROOM to move it's head...which should (in theory) move it's hip...now sometimes (especially with green horses) you might need to tap your horse's hip with your lead or a dressage whip so that it understands when you move to the right (getting out of your horse's way) it needs to move it's head to the right and it's hip to the left. Thus making a circle to the left. (see illustration below)
Circle to the right (you will be closer to the cone than the horse), you will actually use your body to move your horse's head, you will not be touching your horse with your body, but more like using it as a block to guide your horse, and when the head moves (in theory) the hip should move. See illustration below.
What this accomplishes is smooth arcs and circles during patterns and if you need to back a straight line and notice your horse is moving it's hip in one direction or another, a slight change in your body by either moving away or into your horse should correct your error and straighten out your horse.