Passive shoulder external rotation in neutral

Lie down on your back on a mat or bed.
Relax your affected arm in the sling and rest it by your side on a pillow.
Use your good arm to lift the forearm of the affected arm up off your tummy, rotating it outwards until your fingers point up to the ceiling.
Do not rotate it any further, your upper arm should remain by your side.
Gently repeat the movement 10 times.