I really have nothing more to add to what has already been said, but make it an everyday thing...to set up and stand; when you are coming in and out of the barn, on the cross ties etc. I show AQHA SMS and halter, and by making it a daily routine....my horse will set up when I look at his feet.
If you are just doing halter, you can set the feet yourself....there are no rules about touching your horse in halter. Always set the back feet first, then stand back to see if you have a straight line from the point of buttock, to the hock, then to the ground. Then set the front feet with your hand if you have to, and make sure the front legs are straight down from the shoulder. When teaching a horse to square, I will say WHOA each time a foot is placed correctly.