I am in the process of teaching a client's horse to bow for mounting because she had a has a bad hip.
I started out just having him follow the treat to his ankle, then a little more each time so he got the idea of rocking backwards. Then I tapped his leg for him to lift it, then once its lifted I lower his head and rock him back with him following his treat. If he went to put his leg down, I'd tap it up again.
In the beginning he'd snag the treat and shoot his head back up, so I made a point that his head must remain down even after recieving the treat. Little by little the horse gets more generous and will learn to go down on one knee..here's his progress so far.