Side-Lying Dumbbell External Rotation

Trains the Shoulders, Infraspinatus and Teres Minor.

How to do it

Lie on your side on a bench or the floor, with the arm you are training on top.
Hold a light dumbbell in the top hand and tuck that elbow against your ribs, bent to ninety degrees, with your forearm resting across your stomach.
Keeping the elbow pinned to your side, rotate your forearm upward and outward, as though opening a gate. Your upper arm should not move.
Raise as far as the shoulder allows without the elbow lifting away from your ribs — usually somewhere near vertical.
Lower back under control until your forearm is across your stomach again.

Letting the elbow drift away from the ribs. Once the elbow lifts, the rear deltoid starts doing the work and the movement stops being rotation. Holding a rolled towel between your elbow and your side is a crude but effective way to enforce it.
Using too much weight. The infraspinatus and teres minor are small muscles with short leverage, and a dumbbell heavy enough to feel substantial is a dumbbell heavy enough to recruit everything else. Two to five kilos is a normal working weight here and that is not a sign of weakness.
Swinging the arm up rather than rotating it. If the shoulder or the torso moves to start the repetition, momentum has replaced the muscle. Slow it down.
Assuming that pressing already covers this. External rotation strength is typically a fraction of internal rotation strength, and that gap widens in anyone who presses a lot and pulls a little. Nothing in a normal program rotates the upper arm through a range, which is why this small exercise exists at all.