The first thing you should do is to find a safe height
, like a stable ladder, a tall ramp or what have you. Let the horse stand still and then climb the height in front of him. Carry with you a treat and have him smell it before climbing. When you're on top bribe the horse and say up', encouraging him to raise up to get it. Do it repeatedly and reward the horse every time he gets it correctly. Let him only jump 2-3 times every time you give the cue word (up) so as not to tire him or get him out of balance. After he succeeds a few times, let him rear without any treat in hand. Have him practice rearing with only the cue word. That's how to train a horse to rear without much risk.