Cut off the sleeves and bottom of your shirt. You can also cut off the collar.
Save the strip of the fabric that you cut from the bottom of the shirt.
Cut a V in the back of your shirt.
Pull the sleeves together in the back, tie a knot with the strip of fabric and start wraping down.
Knot it at the end. Don't worry about unclean edges, when you wash it they will curl in.