always, always warm up.
if you're serving from a hopper then take breaks after every 50-75 serves. a few minutes gathering balls or something that doesn't involve using your shoulder. don't start doing forehand drills. Seeing as you have had past shoulder issues, maybe the ten serves, then gather balls and switch sides is a better way. I've started doing this recently even though my shoulder is healthy as i'm playing tennis more regularly now and don't want to over-work it.
also, get a good coach to make sure that you're using the proper form. taking a video of yourself will help you identify form issues as well.
get your shoulder checked out. do you do physio?
As to how to practice, I do consistency drills and/or placement drills (splitting the service targets into three to mimic shots to the backhand, into the body and to the forehand).
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM