Looking at the condition of that fender, I believe that you can skip practicing on scrap metal. Those are so badly damaged that I can barely tell what they go on. The only clue is the two rivets for the rear fender support bracket. Your labor is free, so you have all of the time in the world to spend on that fender. If you can't get it to straighten out and end up making it worse, you won't be out much at all. There are better used fenders available which are much easier to make nice.
I still encourage you to give this fender a try. You will learn a lot about sheet metal work. You have received a lot of the information which you need to work a dented fender.
Go for it!
