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 1: Apply to the Fabric of Your Life
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.