Begin by placing an exercise band high up in a doorway. Stand facing away from the door. Grasp the band with the arm you wish to exercise and lift your arm up to the side until it is parallel with the floor. Bend your elbow until it is at 90 degrees with your fist pointed toward the ceiling. Slowly rotate at your shoulder, bringing your hand forward and away from the door, keeping your upper arm parallel to the ground and your elbow bent to 90 degrees. Turn around and perform facing the other way for external rotation. Perform both together for best results.