“How do you stretch a shoulder impingement?”

If you are confident that you have a shoulder impingement then towel stretches are the best in my opinion. They may hurt but your shoulder will feel better. I would avoid flossing and use a static stretch for as long as I can tolerate moderate pain. https://www.youtube.com/watch?v=-YFWrYkJVBs

It depends upon which muscles are contracted, forcing the bone to invade the neural pathway. The stretch must address the origin and insertion of the offending muscularity and correspond accordingly.

Shoulder impingement is a very broad category. In most cases, you have to stretch guarded muscles that are too tight and strengthen muscles that are too weak.

Shoulder impingement happens because your subscapularis muscle isn't working properly. It's going to be tight and weak every time. Stretching and exercise don't fix the problem however. The muscle needs loosen up by making more cellular energy which can be accomplished with laser therapy. I work with the person on their back and their hand just above their head in a comfortable position with elbow bent at about 90 and shoulder in a comfortable position at about 45 degrees to the body. Point being I have access to the arm pit. That's where the subscapularis resides because it's in front of the scapula between the scapula and rib cage. Now in this arm position the person often cannot touch the elbow to the table and if they force it, the impingement pain gets worse. As I loosen up the subscapularis they can get the elbow down farther and the impingement let's go. They can always tell within even ten minutes of lasering that their muscles are loosening and that the armpit is what needs to be loosened to resolve their impingement. This is the first time I've used findatopdoc to answer a question so hopefully you respond and ask another question about this topic.

With an impingement of supraspinatus, stretch the pec and upper trap and strengthening mid and lower trap and rhomboids.

Generally, a doorway stretch for the pecs and a gentle active range of motion of the shoulder help relieve some of the tension. A tennis or lacrosse ball for the external rotators of the shoulder may ease some symptoms too. Hope you find that helpful.