The same as any other horse, only respect is even more important since she is so big. You're on the right track with making her lead right. For your own safety, you may need to move to a rope halter or even have a stud chain handy (just because she is wearing it does not mean you have to use it or be harsh with it) to make sure she's not running you over. I'd get her solid and responsive in the halter, maybe do a little longeing, then tack her up and get on. I bet she goes right back to riding like a champ.