Vinegar (quite a lot)
A spray bottle
The process works best on a solid-colored cotton or cotton-blend fabric. Black is the most dramatic but can be unpredictable: some blacks bleach to gray, some to brown, some to red.
Your end result will be a silhouette printed against a background of mottled and marbled bleach-splatter. Fine detail doesn’t always make it, so anything you can reproduce by sketching with a Sharpie or other blunt felt-tip is best.
Draw your design out on the DULL SIDE of the freezer paper. If you’re adapting a design, you can trace it with the Sharpie. (Note I say “adapt,” not copy. Copyright laws exist, so stick to the Fair Use guidelines.) Cut it out (embroidery snips are useful for narrow parts and interior cuts) and iron it onto your fabric, GLOSSY SIDE DOWN. (T-shirts are an inexpensive practice material and will give you unique leisure wear.)
Check the placement--you can lift and reposition it once or twice without harm.
Now place the fabric on a waterproof surface (especially important for a shirt--insert plastic between the front and back) and spray it liberally with a mixture of one-third bleach and two-thirds water, freshly mixed. Interesting effects can result from adjusting the coarseness of the spray, but if you use a FINE spray, be sure to cover your mouth and nose. Breathing bleach is neither fun nor healthy.
You will be able to watch the color fade in a matter of seconds. Touch up any missed areas and watch them develop, then take the entire thing to the sink or shower and rinse off the freezer paper. Once the paper is off, plunge the fabric into a bucket of half vinegar, half water.
Wring out and wash as usual.
Thus far I have made his-and-hers Raven shirts for us, as well as a "cascading guitars" print for him and an equine silhouette for me. T-shirts are cheap, bleach and vinegar fairly so, and one roll of freezer paper will last you for dozens if not hundreds of designs.