HOW: Lay on your stomach on a bench or table with an arm hanging down at your side. Keep your elbow straight and use your shoulder blade muscles to bring your arm up and out at a 130 degree angle from your shoulder with your thumb facing up (think of making half of a Y with your arm). Once your arm is at shoulder height, return to the starting position and repeat for the prescribed amount of reps.
FEEL: You should feel your shoulder muscles working.
COMPENSATION: Keep your upper trap relaxed, only use your shoulder blade muscles.