What you need: A halter, 2 lead ropes, and whip (a bat works the best or a dressage whip), treats help to.
Step 1. Lunge your horse.. Its better to try with a tired horse.
Step 2. With your extra lead rope (one with out a lead shank) loop it around your horses pastern so that it loosens when you loosen the rope.
Step 3. Cross it under the horse to the opposite side and then over the back
Step 4. Tap your horse just below the knee until your horse picks up his leg and hold the rope tight so that it holds his leg there and then give him the treat and then release. Do this a few times till he gets the hang of it.
Step 5. Repeat step 4 but try to get your horse to eat the treat from between his knees and keep teasing him back till his knee touchs the ground. Let his knee stay on the ground for 3 seconds and then release and give him the treat!
What ever you do don't let go of the rope when your horse pulls back and starts acting stupid...
Sorry it is really hard to explain but I tried!