Because the tutorial that I used to start making these actually seems to have vanished off the face of the internet.
Freezer paper is awesome - I can't believe I never heard about it until now. You can use the iron-on stencil for fabric paint instead if that's your bowl of fish... I just thought the bleach idea was neat as a cheap sort of thing.
You TOTALLY want to do this in a well-ventilated area. I had all my windows cracked open and I was still high as a kite by the time those shirts were done.