Lie on your back and flatten your shoulder blades against the floor.
Hold a weight in one hand.
Whilst keeping your shoulder blades still, slowly lift your weaker arm up, leading the movement with your thumb.
Keep your elbow straight and point your arm straight up towards the ceiling.
Lift your shoulder off the floor by reaching your hand up towards the ceiling.
Hold this position before returning your shoulder blade to the floor.
The movement should come only from your shoulder blade so do not allow your body to rotate.