For this trigger point technique, use a use either a lacrosse ball or softball.
The sweet spot is under the posterior delt and above the teres group, where the ball can come in contact with as many muscle bellies as possible, but also with the suprascapular and axillary nerves as well. This is the major landmark you'll want to hit.
Lie on your back with your feet on the ground and with knees and hips bent. Your ball will be in direct contact around these nerves in the back of the shoulder. From this position, apply a 6/10 relative pressure into the ball while slowly moving the arm into controlled elevation.
Move the arm just enough to create some relative gliding actions between the muscles and nerves under the ball's contact point. Concentrate 15-45 seconds here.