If your horse was trained for any showmanship, he should know how to do it, at least from the ground.
I'm horrible at explaining aids, but I'll give it a try: when I ask a horse for a turn on the haunches, I stop him (usually along the rail of the arena, especially if it's been a while since he's done it), sit deep, and use my outside leg at the girth to ask him to turn. I keep my inside leg off of him and use my seat/weight aids to keep him from moving forward.