Start lying with the band directly above the shoulder and so there is tension when the arm is at it's maximum abduction range you can go (it's not slack). Have you arm to the palm is facing up. Pull the band down and outwards then down to your side using the muscles under your arm pit. Slowly then raise your arm up over your head in a outwards abduction arc, letting the band take the load of your arm and drag your arm overhead. Back sure for the first part of the movement your shoulder blade stays down, and then rises once your hand is in line with your head and you make sure your scapula elevates, abducts and rotates with the arm. You should have tension in the band at the top.