Lie on your back with a pillow supporting your head if required.
Fully bend your elbow on your affected arm.
Use your other arm to lift your affected arm up, keeping your elbow fully bent.
Once your arm is at 90 degrees with your body, straighten your elbow.
Your arm should now be fully vertical.
Keeping the assistance of your supporting arm around your wrist, begin moving your arm down towards your side.
Move your arm back to the starting position and then up towards your head.
Start with very small movements and increase the size as you are able to.
You should not have pain.
You may eventually be able to move your arm all the way from your side, up and back to lie by the side of your head.