First, find two too-short tshirts that coordinate. Preferably they should be a similar width at the bottom. (My gray one was wider so I actually took the sides in a bit.)
Once you've decided which will be the bottom layer of your fooler shirt, evenly cut a 2.5" - 3.5" strip from the bottom of the tshirt. I used my rotary cutter and mat to make it really easy. You'll be using the existing hem.
Stitch or serge together.
Flip the under layer down and refold the hem of the tshirt, with the seam facing up, and pin through both layers.
Topstitch - I used a 4.0mm double needle to topstitch for a professional-looking result.
Voilà! Your lengthened hem is finished!