I taught my horse to bow by having her reach for horse cookies that I held between her front knees and simultaneously giving a voice command.  At first, I had to help her by having her follow the cookie (with her nose) to that location before I gave it to her. Over time,  I held the treat lower between her front hooves. All you need is repitition and consistency. You just have to be careful to avoid allowing your horse to become pushy when using treats. Also, only attempt to teach this to a gentle, well behaved horse, for obvious reasons.  Bowing also helps stretch their neck and shoulder muscles.

Good luck & have fun.

I used to ride at a barn that taught one gelding how to bow.  Couple years later he was sold and the new owners found out he could bow when he did it in the show ring when lined up waiting for the judge to ask them to back.  The new owners weren't too thrilled, but the crowd loved it!