Once you have mastered strengthening your shoulder in one single plane (flexion/extension, abduction/adduction), you can challenge your shoulder with PNF exercises. PNF stands for Proprioceptive Neuromuscular Facilitation. This is when you challenge your muscles in different ways and different planes.
The first motion is called a D2 pattern. Cross your arm over your body and turn your hand like you are grabbing an imaginary sword out of your pocket. Pull your arm up and across your body while turning and opening your hand until it is up in the air on the same side as the arm with your hand and fingers fully open, like your are throwing the imaginary sword up and away from you. Now reverse that pattern going back down.
Then you will perform a D1 pattern. Make a fist with your hand, put your hand across your chest, almost like you are saying the Pledge of Allegiance with your fist closed. Now bring your arm down and back across your body while twisting and open your fist behind you. Then reverse the pattern back upward.
Once you have mastered these, you can add in a resistive band, but remember, these are very difficult, so don’t do these until you are ready!