Step 1: Controlled Bleaching with Discharge Paste
This is more of an introduction to discharge paste than an instructable. Yes, the name is terrible and the stuff is white and gooey which makes it even more wrong. But it's pretty damn cool, so you just accept it and move on towards greatness.
Where bleach is incredibly thin and can destroy natural fibers if you're not careful, discharge paste is the opposite. It's gooey so it doesn't spill easily and can be used with silkscreens. It's also nice to natural fibers, which is good as well. It removes most fiber reactives, direct dyes and acid dyes and typically leaves a light golden color when it's done.
Everything you need to know about this is printed right on the label. To use it, you just apply it to a fabric, let it dry, and then iron it on the lowest steam setting for a few minutes to activate it. To make sure it works on a new fabric, test it by putting a small spot in a hidden area of the target material (assuming you're using a shirt) and see how well it works before spreading it everywhere.
Available in the U.S. from Dharma Trading Co.
Step 2: Apply to the Fabric of Your Life
Put down a stencil or a silkscreen or forgo all premade plans and apply the paste directly to the fabric. If you want to let the goo seep in a little deeper, you can thin it out with some water. You can just go for it or keep on testing on more spots on the fabric.
Used shirts are cheap to play with, but for consistency and thin shirts, go to the underwear aisle and get the dyed t-shirts. They're usually less than $5 each for a decent one and tend to be thinner than the Beefy T's that make me sweat like I still live in SoCal.
Step 3: Make Magic with Your Iron
At first there's not much to see on the shirt. Even when you wait until the paste dries, there's still not much to see. This all changes dramatically when you put the iron on the fabric. The color quickly disappears and POW! there's your design.
One more thing, be sure to be in a well-ventilated area. This stuff stinks and when your fabric is done it too will stink until you wash it. So don't run out to a bar to show off your new clothes to your friends because you'll destroy your shirts first impression on them.
Now clean up your mess and plan out your next project.
Step 4: BONUS! Before and After... Again.
One more example of how the paste looks dried on black cotton and right after it's been ironed.