I always try to think of “shoulder blades together”. This is always the best way to fix my posture and for some reason it makes more sense to me than any other tip. Imagine that there is a string attached to each of your shoulder blades and it is pulling them together, and another string from the middle of your shoulder blades down to your butt. This seems to help me keep my arms in place as well. Good luck!
Sit deep in your saddle and try to tuck in your elbows beside your waist but still give with your hands and try as hard as you can to keep your hands still
Not an expert or anything! Maybe try and work on your balance so you don’t have to use your hands to balance. Sit down and deep to try and keep your shoulders back. To work on balance maybe search up some excercises to do!