Stand up straight with a wall to your side.
Your shoulder to be exercised will be closest to the wall.
Take two tennis balls taped together and place them on the wall by your shoulder.
Lean onto the balls with moderate pressure and take your arm up and across your body.
Holding your arm in this position with your other arm, rotate your body away from the wall, then back towards it.
The balls should roll across the side and back of your shoulder.
Increase and/or decrease the pressure you place on the balls according to how it feels for you.