Great advice here already. I’ll just add (and I don’t know if I’ve missed this already mentioned by someone)… a racehorse expects to use the rider’s hand to balance ON. They expect to take a “hold” on the rider’s hands. They pull against that “hold”. They gallop under that “hold”. It makes them into a “coiled spring”. When asked for full speed, in a time workout or race, the “hold” is lessened, not fully released until full extension is required… down the lane to the wire. Also, the rider has very little “leg” to use on the horse. Stirrups are usually shorter than with non race riding, vary from “short” to “very short”. The horse is cued forward with a chirp from the rider, or the stick. When being broke to ride, the rider does more use leg cues as a normal riding horse, but once at the track, not so much. So, though the horse has plenty of “forward” under many circumstances, he is not well accustomed to moving off leg pressure. This usually has to be “re-taught”. And if you pick up any speed, he will try to take hold of the bit and tow you. This is why some people say about riding OTTBs, that if you pull on the reins, they go faster- it’s not exactly true, but it can feel like it. Some hit the show ring without ever being “rebalanced” correctly, thus the issues that some people have with riding an OTTB. Rebalance the horse first, early in the re-schooling process… that is, teach him to engage and use his hind end, and not balance on your hand. Teach him about subtle leg cues. Don’t let him use your hand to balance ON. He must carry himself, not lean on your hand. If you don’t teach him these things, he will remain a racehorse.
Other than that, it’s just a matter of a simple re-breaking job. Showing the horse what you want him to do for you, getting to know each other, interacting together. Some if it will be new to him, some of it will be new to you. Either you work it out successfully between the two of you, or you don’t and it’s a disaster.
Depending on where he comes from, he may or may not need some time to “let down” off the track. Then you start getting to know each other, grooming and ground work. Find out if there are any holes in his training that you need to fill for him. Lunging, walk, trot, canter (NO side reins- he will lean on those). Work on relaxation in his work, suppleness, and response to your cues. Form a partnership. Become friends. If you can’t do this yourself… well… then he will form the relationship with someone else, not you. He may be able to extend what he learns from someone else to form a similar relationship with you, later. That’s often what horses have to do. BUT, I agree that it is a “learn by doing” sorta thing, and it ain’t all that difficult. A horse isn’t a “machine”, he can’t be “programmed” by someone else, then you get on and press the buttons. It’s always a relationship, a mutual understanding (if it’s any good). JMHO. Learn how to do this, it’s part of the joy of owning an OTTB. Good luck.