Lay your t-shirt flat, facing up. It's ok if your shirt isn't this cool; the technique will still work
Pinch your t-shirt with your right hand. About halfway down, even with the left edge of the left collar. Make sure to pinch the front and back layers of the fabric.
With your other hand, pinch the left part of the collar, then fold the shirt with that hand... Eh just watch the video :P
Behind the scenes!
Repeat ad infinitum