Start in a kneeling hinge with your hand lightly pressing a swiss ball to the wall. 'Scoop' your armpits forward to engage your scapula stabilisers. Maintain this scoop (there should be space between your ears and shoulders) as you move your chest through your arm to end up in an overhead position. Avoid pressing into the ball too much to begin with and you should feel the muscles around your scapula working to stabilise. To make this harder, draw circles while maintaining stability in the scapula.

Shoulders, Mid Back

Overhead Push