It took me a month to get my gelding to where he is now, and I don't even do showmanship! I was just teaching him for trail class with the pivot box.
Anyway, I started off by taking a thick stick and wrapping a cloth over the tip so it wasn't pokey. Then I would pull my horse's head kind of towards me so gravity took over and he automatically took a step with his front legs to the side. At the same time I did that, I prodded him behind the elbow with the stick. If he moved the back foot out of place, I stopped, quickly led him forward a step so he put the hoof back in place, and as soon as that hoof was in place, I patted and praised him.
I eventually replaced my hand with the stick, and then transferred it to the saddle. When on the ground, I don't have to put my hand under my elbow anymore, I just walk sideways in the circle and he'll do a full pivot.