This is the method I used...
It's not so hard to teach them to go onto one knee, and it can be alot of fun! I did it with my haflinger using carrots cut into little pieces.
For starters, tie a rope around his left foreleg. Tap the front of his forearm and when he raises it, if only for a second, reward him. This doesn't have to be perfect, just do it enough so that he gets the idea. This will eventually be your que ^^.
Once he lifts easily when you tap, hold his leg up with the rope. Due this a few times and reward him when he stands still or relaxes, because we don't want the rope to make him nervous. Once he's okay with you holding his foot up, take a carrot and bait him to lower his head towards the middle of his front legs. As soon as he leans back even a little on the front leg that he's standing on, release the pressure and reward. Let him stand on all fours for a sec and think about it. Then repeat. As he gets it, ask for him to lean further and further down.
It might take a little bit for the idea of putting their knee on the ground to "click". My horse was almost doing a split when his knee started to buckle a little and bend on its own. I imediatley took the pressure of the rope off and rewarded with a carrot piece. He would then bend a little more and a little more till his knee hovered above the ground. He finally buckled a little and "touchdown!" He was graciously rewarded with a -whole- carrot. This is when you'll want to give him a nice little 60 second rest the think about
His accomplishments and then pamper him with praise and scratches. My horse was able to get to this point on the first night that I worked with him, but every horse will be different and yours may take longer. Or quicker. Who knows?
Once your horse finally does touch the ground, don't reward him until he does it again. Soon your horse will realize that the treat will only be dispenced if his knee is touching the ground. Once he kneels confidently, your can take the rope off and start guiding the hoof with your hand, since by now he gets the idea and won't try leaning on you as much. Don't forget to always pat his leg before you ask him to do this or else he'll never quite understand the que. You can also start adding a voice comand if you like. You also want to add a release comand. Such as "up" or "okay" or whatever you want it to be. Don't expect him to understand it just yet, but say it just after he gets his treat on the ground just familiarize him with it. This is so that when you eventually go to mount him, he'll stay down till you say so. ^^
Eventually start using as little support as possible with your hand. You want him guide his foot on his own. Just start keeping your hand near his leg and correcting him when he needs it. Eventually you'll just be guiding him with your carrot and he'll do the rest. Now we have to wean him off of the carrot. This is where your release word comes in. ^^
Now, when you ask him to bow, bait him down with the food as usual but ask him to stay own a little longer. Maybe 2 seconds, then 4, then 6, 8, 12, 16, 20, 25, 30 and so on. Increase the time slowly. We will also no longer be rewarding him while he is in this position. We will ask him stay there for 2, 4 , 6 seconds. Then we will say our release word. When he gets up, -then- we reward him with the carrot piece. We are now establishing that #1 he will not be rewarded unless he stays in position till we tell him to move, #2 the carrot he sees when he performs the trick, wil no longer be the carrot he receives, thus an end for need to be baited with food, and #3 that the trick must now be completed, before being rewarded.
Linger a while on this stage of the training. Make sure he knows what the release word is. If he doesn't listen to the word and gets up anyway, don't punish him, just don't reward him either. Ask him again and when he listens, then reward him. If he doesn't want to stay down then maybe you are asking for too much at once. Lwoer the amount of time that he needs to stay down just temporarily. Make the bar low enough to for him to jump... theoretically. If he doesn't succeed often enough, then he will lose interest.
Once he understands this completely, then start baiting him down without food in your hand, and then eventually without baiting him at all. If you want it to look fancy, then start rewarding him only when his head is tucked down. Once his head is in or near the position you want it to be, then give him the release word and reward him.
Once he has this trick mastered, and I mean mastered, you can start to wean him off of treats entirely. Give him a carrot more and more on "occasion". And vwala! A little practice, patience, and some carrots is all you need!
Now I'm not saying this is the only way to do this. I'm sure that there are ways to teach this without the carrots and what-not, but this method has worked very well for me and my horse. It's a fun thing to do in the stall on your down time, too. I'm going to apologize right now for any typos or spelling errors in my mini book that I somehow managed to type, But my brain is too fried to proof read, so good luck! I hope you find a method that works well for you!