HOW: In a side-lying position place the arm facing the ceiling slightly abducted, this can be done using a pillow or towel. Now focus on purely rotating the shoulder from the hand near the stomach all the way out to where the hand is reaching towards the back will. This motion should be initiated with the shoulder blade being pulled back followed by the arm pulling back. To make this exercise more challenging, you can add a resistance band or a dumbbell.
FEEL: The muscles in the back of the shoulder should be working here.
COMPENSATIONS: Avoiding stabilizing at the scapula first. Many will compensate with straightening their elbow as they rotate out, keep the elbow bent to about 90 degrees for this entire exercise.