First things first, you decorate the front of the shirt. Display your hatred of fan-girls or whatever you choose to write. I did mine in sharpie, but you can embroider if your choose.
Cut off the sleeves and neck line straight across as if you are going to be making a tube top.
After you get your writing all situated, you make up a pattern for the shape of the shirt. If the shirt already fits you, more power to ya!
After cutting and sewing the sides so they fit you, finish the top of the front and back as well as attach the straps with some scrap material.
VWALA! You have a t-shirt.