Finding your seat in the saddle by riding without stirups.
With any saddle work you will need to make sure you have a correctly fitting saddle and that it is placed correctly on your horses back. One of the reasons I like the dressage saddle is it keeps my horses shoulders free which is most important for my Missouri Foxtrotter but also important for all my horses as well. The seat of the saddle should be level horizontally from a side view.
Have someone help you by taking photos or a video to see if you are in the proper position with your ear, shoulder, hip, and ankle in a straight line. It is important you do not have a “chair seat” where from the side view it looks like you could be sitting in a chair. Picture that if you removed the horse from under you whether you would still be able to stand on the ground without falling. If you are sitting in a chair and the chair was removed then you would fall. Stand on the ground with your feet slightly wider than shoulder width apart and pointing forward. Squat down a little and open your knees a bit. Your pelvis will be tipped slightly back. Open your chest with your shoulders softly back and down. Head looking forward and not up or down. That is a nice dressage position. When on a horse your arms should be soft and supple. From shoulder to elbow will be relaxed at your sides and there should be a straight line from your elbow, down the reins, and connecting to the bit. If your hands are too high or low the straight line and connection are broken.