Shoulder internal and external rotation supine with weight (Shoulder external/internal rotation strengthening, with dumbbell, arm out to side, 90/90, supine)

Lie on your back with your symptomatic arm stretched out at approximately 90 degrees from the side of your body holding onto a weight.
Bend your elbow to a right angle and then slowly rotate your forearm up and then lower it back down, keeping control of the weight.
Make sure your shoulder blades remain in contact with the floor throughout the movement.
Relax and repeat.