No stirrups is one of the best and most simple ways to gain balance. Sometimes my trainer has me flat and jump with one hand holding the reins and one hand straight up in the air. This has helped me develop a good seat and better balance:)
Hi Sharli, I would suggest to work well your balance on flat work before. Then ride without stirrups above poles and small jumps. You can also do schooling lines without one hand or with your eyes covered.
Hope this helps :)
A lot of grid work is helpful, working without stirrups (in moderation so you don't encourage incorrectly squeezing your knees), and going on a lunge line and riding without reins- this will really help gain balance and develop your seat.