Watch he Trocha videos, lots and lots, you'll learn something new every time. But if you want instruction on here, I'm bored, so I'll give it a shot
1. Make sure he's light and supple for best results. (doesn't pull on your hands when you stop or back up) It helps alot if you can move his hind over too. You'll use this to correct turnarounds.
2. Walk circles while tipping his nose to the inside. spiral down until your horse is turning as tight a circle as you can get.
3. Anytime you feel him moving forward instead instead of turning around, you can back up. Not alot, just to the point where he started forward impulsion. After you back, start your turn again. Continue turning and every time you feel forward moion, bring him back and start another turn.
4. He may start locking down and not turning, kind of like a horse backing a few steps and sulling up. If this happens, you may be rushing them but what I do is slap them on the indirect side with your rein.(turning left, slap on right side) If you do this, make sure you're telling them where to go with the direct rein. If you don't do this correctly it will really screw him up.
5. Don't start asking for speed until he's properly turning around. When you do, use the outside leg and use the spur for reinforcement only.
I'm NOT a reining pro, but this is how I do it. I skipped lots of details, but this is it in a nutshell