Always start your routine with the flexibility section as your warm-up and the easy strengthening as your beginning point. After a few weeks, when you feel confident with the easy routine, then add the medium strengthening routine. Only after several more weeks of mastering the medium routine should you proceed by adding the difficult strengthening routine. In any case, ALWAYS start each session of exercise with the flexibility routine as a warm-up.
Always start your routine with the strengthening exercises (flexibility) before proceeding to the strengthening exercises. This routine says: "You should not feel pain during an exercise." Pain is relative. Stretching can be hard and yes, you can feel discomfort! Be smart, you should not feel severe, or undue pain during your exercise, but no discomfort likely means you aren't doing anything. Rehabbing a shoulder is going to be a little uncomfortable. If you feel undue pain, talk to me (your doctor) about the specific exercise that is causing your pain.