Unfortunately riding without stirrups along with a good instructor who can put your legs in the correct position and then continue to keep correcting them as you ride. Having some lunge lessons at a riding school on a horse which is used to this can also be a huge help as you can just forget about having to steer and concentrate on staying on!! Good luck with this. It hurts like mad at the time but is well worth it in the end.
You know what Andrew Nicholson says? 15 minutes every day without stirrups. I suggest you do it on the end of a lunge with a good instructor to start with, who will keep hold of the horse and adjust your position.
I’ve started warming up my horse without stirrups just while we are in walk and again at the end of the session to allow my legs to stretch. He is an older horse and not spooky so I wouldn’t take my stirrups away on a diffrent horse.
Work on the lunge is good if you can find a suitable horse / instructor. I had a session on a mechanical horse and it was great for figuring out a few bits and bobs without disturbing a real horse. Pilates might also help.
Another vote for riding without stirrups. And Equi Pilates - this could help you improve when not riding, particular if you use the ball while watching TV.