okay, so the first trick is to teach your horse how to JUST move their rear end around.
thats easily done if your horse yields to pressure.
first you must start out on the ground. move the rear end over then move their nose over. its pretty much like the sideways game that pat parelli teaches you in natural horsemanship. once you've got the hind nose thing down, think of a good cue [the best would be where your foot would be while riding]
You can tell a gelding, you can ask a mare, but you must discuss it with a stallion.