Lie on your back and flatten your shoulder blades against the surface.
Whilst keeping your shoulder blades still, slowly lift your weaker arm up.
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 back against the surface.
The movement should come only from your shoulder blade so do not allow your body to rotate.