The posture is a combination of the stance and the body position. By using a good posture, you build a strong platform to support the rest of the shot process.
The stance can be a parallel stance or an open stance; but the open stance is recommended. When the open stance is used with proper body position the abdomen can sense when the body is leaning and then we can correct our body position. Also, the open stance rotates the head enough so aiming is easy.
A parallel stance is easy to set-up. With one foot on each side of the shooting line, place the feet parallel to the shooting line. The problems with the parallel stance are the body can lean and the head is not rotated towards the target enough.
An open stance is a little harder to set-up, but provides much better control. Start with the parallel stance then turn to the front of your body. Next, move your back foot, the one farthest from the target, forward by ½ foot length. After you move your back foot, turn your body, including your feet, legs, hips, torso, shoulder, neck and head, towards the target by 30°. After turning your body, rotate your shoulders so they point just a little bit in front of the target.
Once you have your stance, then you must position the rest of your body. The back must be flat by pulling the navel to the spine and tucking the hips underneath your spine. (If you know what a pelvic tilt is, that is exactly what you are doing.) Next, drop your chest about 1 inch. Lastly, place your about 60% to 70% of your body’s weight on the balls of the feet with equal weight on each leg. To move your body’s weight, push the head slightly forward, approximately 1 inch. If a line is dropped from the bow side ear to the floor, the line would clear the bow hip and strike the floor in front of the bow foot.