I taught mine for 4H showmanship and am teaching my new one just for the respect factor to stay out of my space and to be a touch more subtle with his big feet.
First, make sure he will back up when you tell him. The word "whoa" is also great for them to know.
Halt the horse and start by just practicing moving those feet in the direction you want, when you want via backing or stepping forward, whichever is needed. As he gets the idea, use "whoa" to shorten each step until you can place them where you want.
Don't work one it much more than a few (5-10) minutes at a time, though. It can get very frustrating for a horse that doesn't understand it and then he'll be even tougher to teach and he'll learn to hate it.
Hope this made sense.
"Radar, the man's ex-cavalry: if he sees four flies having a meeting, he knows they're talking about a horse!" Cptn. BJ Hunnicutt, M*A*S*H Season 4, Episode "Dear Mildred"
and it's going to depend on what show/division you want to do.
Hunter conformation at Upperville is a different thing than Halter at Congress.
Hunters are to stand in the "open" stance which is one fore leg slightly back, one hind leg slightly forward and the weight even on all. The idea being the judge can see all legs at one time from both sides.
I find if I turn right in front of my horse as I say WHOA, they generally stop correctly. May take some tweaking to adjust a foot slightly and moving forward somewhat easier to adjust.