I saw a project like this on pinterest and so I thought I would give it a try.
My husbands Large t-shirt
With scissors I cut the sleeves off, cut down the center and also removed the collar. When I cut down the center I removed about 3-4 inches of material in the middle.
You also could sew the edges but if you don't mind a more none perfect look you can just leave it. I suggest using shirts without a pocket because when I went to remove the pocket my shirt ripped in one spot. So I either have to put a patch or just start over with a new shirt. But since this project took me about 10 minutes I think I could manage just making a new one.