Sitting: shoulder blade sitting too forward (Scapular position too forward - retraction, sitting on ball)

Sit on the ball.
Tilt your pelvis forwards and back until you find the mid position where your low back feels comfortable.
Keeping your arm relaxed, gently draw your shoulder blades together so that your shoulder moves just a few millimetres backwards.
Hold this position.
Keep the muscles between your neck and the top of your shoulders relaxed.