Lying on your side on a bench with your top elbow tucked to your ribs, you rotate a light dumbbell up and away from your body, then lower it back down. The strict elbow position keeps the work isolated to the small rotator cuff muscles instead of letting the shoulder take over.
Log your dumbbell side lying external rotation (on a bench) sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.