I am the first to admit, I know very little about training horses. Okay, virtually nothing. But according to the advice I've been given by my farrier and by various other horsey-type people, until a horse is two years old or so, training consists mostly of introducing them to all sorts of stimuli so they are not spooky, and of getting them habituated to being touched all over, groomed, et cetera. And of course learning to walk on a lead and to load in a tailer, and that sort of thing.
Then I put Paloma on her back. She really didn't seem to care at all, or even notice. However, Paloma is not such a great balancer, so I took her down after only a minute or so.