rock with the horses movement so instead of going up and down try to find the horses movement going side to side
no stirrup work and use a bareback pad while learning so you dont hurt the horse's back. it's better to be on the lunge line while learning so you don't balance on the horse's mouth. use a neck rope around the horses neck as well so you have something to hold onto if you lose balance. it takes lots of time & practice as well as a strong seat!
Try to keep your leg under you
I would start out just walking and do it for a long time and often so you can start to feel the rythym of your horse. Also riding with no stirrups in the saddle will help you gain muscle for balance when doing bareback :)