This is generally what an untrimmed pork tenderloin will look like when you buy at your local grocer.

Working with the boning knife away from you, start at the end nearest to you, and cut excess fat away from the loin as shown.

Next, trim the silver skin or any membrane left on the loin, again with the boning knife facing away from you.

Go all the way around the tenderloin and trim all sides

When finished, you my have cut a bit off the loin, but it is now ready to prepare