My pony can't really do it at all. So far what I've been doing is halting him, backing him up a few steps, then I start tapping him on the shoulder with a crop, and basically just make him yield away from me and hope he crosses his feet over. I know his back foot isn't staying planted, though. He can't really do any more than 2-3 steps before it gets screwy.
How do you train them to do this?