strengthen your calves!! and thighs and work on stretching your tendons so you can put your heels down further
Working without stirrups or attaching the stirrups together so you have to balance i would work with this a bit on flat first if you do it because it takes time to get used to it :) It will help!
There's no secret ... work, training. You should work without stirrups, it helps! Relax your shoulders, ankles, legs as well
Try to regularly work 5 min on your seat, try to get it as flexible as possible, relax, breath. More your seat will be flexible and the rest of your position will come by their own after many moments of work :)