First get the horse to understand whoa and then work on getting them to just stand still for longer and longer time. All that before worrying about "setting up". Next, your big horse may do this, think as you go what you want to do and time your feet with your whoa (as in whoa 1 [maybe 2 whatever works best], stop). Hard to explain but you must think about stopping and stop your feet so the horse takes it's cue from you. Hestitate a beat with the lead chain firm and slightly tilting the head up. All this helps him to get the STOP. Some will stop squarely just naturally and some you will have to work to get setup square.
After they've learned the stop, you should step out in front of them with the lead taut. Now you can "fix the feet to square. Next use a treat, a piece of hay/grass, your hat (if you wear one), crinkly paper anything to get their attention and get them to follow with their nose. You want to get the stretch just enough that their neck stretches and not their body. If they start leaning forward, they'll either step forward next or just look really silly at an angle so bump down slightly on the lead to get them to put the weight back on their haunches. You want the back as level as possible.
Yes, in Pinto their are people that stretch way too much but don't be fooled. Most of those judges know that's not right. Sometimes you get a judge that's never jusdged minis and they may say some strange things. I had one actually compliment me by saying "Wow, he looks exactly like a 'little' horse" - DUH! Had to bite my lip to keep from laughing!