Keep elbow just away from body, elbow at 90deg and shoulder blade pulled back and down. You can feel the bottom corner of the shoulder blade with opposite hand behind back. Pull the theraband outwards by rotating the arm outwards, pivoting at the elbow as an axis point (make sure the elbow stays bent at 90deg). Don't let the elbow move out of position or let the shoulder blade move - you can use you opposite finger on your elbow as a guide.

Progression: Once you have mastered this try External Rotation at 45º and then 90º.