How do you teach your horse how to bow?

Have your horse in a halter and lead rope so you can guide him if need be.

Start by getting him to reach his head to the ground for a treat.  Then slowly keep putting the treats farther and farther back until it is in between his front legs.  Ideally he should keep putting his head back farther to get the treat but he may start stepping backwards to get the treat.  Don't dispear if he does this.  Just use a halter and lead rope and guide his head down to it. What you are trying to do is let him know that he can reach down and get the treat without moving backwards  Stop this exercise once he has been reaching to his feet with his head consistently.

Now we will work on a completely different thing.  Work on him giving you his left foot,  as if you were going to clean it but instead of facing towards his rear you are facing towards the front.  Get to the point where he easily just rests his foot in your hand.  Ideally he should lift his foot for you but that may not happen until after many days.  Don't worry, we can still move ahead as long as he will rest his foot nicely in your hand.

Now we are going to combine the two and add a cue.  The cue I used was placing my hand on his left shoulder but you can do anything you want for a cue.

1.  Get him to rest his foot in your hand.
2.  Put the treat down between his front legs but now just behind them.  You may need to show him where you are putting the treat by having his nose follow the treat down.
3.  Do your cue.
4.  Now just roll back slightly to his rear end while holding his foot while
he trys reaching down to get the treat.  When you move back about 6 inches he may stop and lean back forward because this is strange to him.  Just dwell for about 15 to 30 seconds and try again.  Do this until he is comfortable with reaching down to get the treat while you lean him back slightly.  After a while of doing this his right foot will be extended out in front.  His left leg will be kneeling down on the ground and his his head will be tucked up underneath him as he reaches for the treat.

Only have him bow down at most 2 to 3 times a session.  Don't come back to a session until an hour or several days later.  You need to have him rest his muscles and think about what he did because it does put a lot of strain on the right leg muscles in order for him to get back up let alone the mental strain.