My method is called the "Tree of Patience", or Clinton Anderson now sells a Patience Poll (same method).
If your horse will stand tied, start here. Go to your local tire shop, ask for a inner tube. Loop the inner tube over a tall tree limb. Tie a lead rope threw the looped inner tube and attach your horse. (This really works best with a rope halter, a flat one is basically pointless for this method.) This way if the horse pulls back, there is some give. Must be high enough if the horse rears up, he will not get a leg over the limb, but not low enough for the horses nose to reach the ground. Tie both horses where they cannot see eachother or tie one where it cannot see the other horse in the pasture. The first day let the horse stand there until he stands still (might take all day). As soon as the horse is not pulling, pawing, etc.... turn him back out to pasture. The next day same thing (usually doesn't take very long the second day) until the horse stands still. Repeat daily, after a few days, do this saddled.
The goal is for you to be able to lead the horse out of the pasture, saddle him, tie him, and him stand there quietly (again I emphasis unable to see the buddy horse) until you come back to get him, whether that be 15 min or 2 hours.
Once this is accomplished start riding the horse alone, headed away from "home" leaving from the tree after being tied 15-30min. ALWAYS LET SOMEONE KNOW WHERE YOU ARE GOING AND HOW LONG YOU WILL BE. If riding on a road, if possible, have someone follow along in a vehicle.
When the horse is riding away from home easily, start riding with a buddy but making your horse move away from the other horse (walk on opposite sides of the road, ride in a creek one horse in the water and one on the bank, make your horse go around a tree to the left when the other rider goes to the right...) just make your horse do something different than what the other horse is doing.
After this is going smoothly, start riding away from the other horse. (After a road ride together take different routes home, take trails that will cross but not riding together, ride opposite directions in your own pasture.)
It can be a quick process or a long one, depends on the horse. It has worked many many times for me and for people I was helping.
One is closer to God's heart on a horse than anywhere else on earth!