Step 1: Grab your favorite t-shirt. Make sure that it's a shirt that is at least a little too big, too small won't work!
Step 2: Cut off the arms, following the seam. Cut around the neckline, as I've marked here. You may cut straight across or a bit diagonally, both work and both are cute. Make sure that you're cutting through the front and back of the shirt. You may cut off the bottom hem as well, if you wish. I pretty much only do this if a shirt is too long.
Step 3: You're almost done already! Stretch out the arms, collar, and bottom (if you cut it) so it kinda curls up a bit and doesn't look freshly cut. Next, cut off the hems of both sleeves.
Step 4: Take the hems you cut off the sleeves and cut in half. You will have 4 short pieces of fabric. Take a piece and tie 2 or 3 knots near the collar on your front strap. Repeat this again on the front, and then on the back of both straps. Done!