First, you need a t-shirt that you like, but that doesn't fit you very well. If you want to be Superwoman, you will need someone to give your teenage son a Superman t-shirt which your son will then refuse to wear. Score! Now your true greatness will be advertised for all to know.
Flip your boxy tee inside out and lay it flat on your work surface. My work surface is the floor -- because Superwoman doesn't need any special equipment.
Make sure the side seams are all lined up. Then get a t-shirt that fits like you want. Lay it on top of the boxy tee. Line the neck and shoulders up.
Grab a pen, pencil, chalk or tube of lipstick (just kidding about that last one. That might get messy, and Superwoman is never messy.) Trace around the side seams of the smaller t-shirt, leaving yourself a seam allowance of about 1/2 inch.
Sew along your seam lines with a sewing machine, serger or you can hand sew if you are really good and really patient. I'm not.
Done. Ta Da!
Now you need to arm yourself with lots of corny Superwoman puns.