how to do shoulder external and internal rotation?

two of the main warm-up exercises that I recommend everyone to do before training any muscle that relies on the movement of the shoulder are internal and more importantly external rotation of the shoulder and there are two main reasons for it. first, by doing these two exercises, you activate the deep rotator cuff muscles that help your shoulder girdle keep its stability. second, they help your joints produce synovial fluid before loading it up with any substantial weight.