How do you retrain a horse to ride?

Treat it as if it's never been ridden. Start slowly getting used to the tack, then get on and walk, try a trot, etc. Make sure to listen to the horse and go at their speed. Do not rush them...If you want a book to reffer to, try Pippa Funnell's "Training the young horse"; even if it's an old horse who's out of practice it's very effective.