HOW: Start on the outside of a door frame or wall. With your elbow bent at your side to 90 deg, push backwards into the doorframe/wall. Think of driving squeezing your shoulder blades to push backwards. 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 back of the shoulder.
COMPENSATION: Do not shrug your shoulders and use your neck muscles.