HOW: Start on the outside of a door frame or wall. With your elbow bent at your side to 90 deg, put your palm against the doorframe and push inwards into the doorway. Think of rotating your hand inwards towards your belly button, with your elbow staying still at your side. Slowly ramp up the amount of “pushing” that you do, staying just under the point of pain/discomfort and hold that level of contraction.
FEEL: You should feel the muscles in your shoulder working, especially the inner parts of the shoulder.
COMPENSATION: Do not let your elbow come away from your body. Do not shrug your shoulders and use your neck muscles.