Here's what you'll need:
-100% cotton, white, low-rise, fly-front briefs (a.k.a. "tighty whities", or "tidy whities", or some variant depending from where you hail),
-a work space to get dirty (outside if possible), and
1 Tie-Dye Kit (I ordered this one through Dharma trading company, at the recommendation of this great Instructable),
Rubber bands, soda ash, dust mask, latex gloves, several colors of dye, and squirt bottles.
A big thanks goes to my friends Bridgette and Lucy for help tie-dyeing and documenting!
Step 1: Prep your undies
Make sure to wash your undies before starting, as new store-bought fabric can get streaks when dyed. Also, if the fabric is damp but not dripping when you start, the designs will turn out crisper.
After you've twisted, tied, and banded, you'll need to soak the items in a soda ash solution before dyeing. This will help the dyes bond to the fabric and keep your undies bright and brilliant for longer. Wearing the dust mask and gloves, dissolve 8 oz. of Soda Ash into a gallon of warm water. Soak the undies for 10-15 minutes, and wring out the extra solution prior to dyeing.
Step 2: Spill some dye
If you're working with powder dyes, hydrate them wearing the dust mask to avoid inhalation. Make a plan for the color scheme, and apply the dyes using squirt bottles, paint brushes, or sponges. For the spiral technique, I divided the circle into three sections: one red, one blue, one yellow. Overlap the sections to create new secondary colors. If you've tied your undies really tightly, be sure to use lots of dye, as a surprising amount of folded fabric will stay white, even when the exterior is saturated.
Note that you should wear rubber gloves during this process, as the dyes will stain skin for days and can cause allergic reactions.
Step 3: Set the Pigment
When they've finished curing, take the undies out of the bags and start rinsing them. A surprising amount of dye will come out, but the cotton fabric should remain brilliantly colored (The elastic waistband, however, will return to white). When they run clear, the undies are ready to be machine-washed under hot water. The kit I used came with professional textile detergent, but I'm not sure that's necessary.
Your undies are almost done, there's only one more step...