Tie dying a shirt with food coloring is similar to tie dying a shirt using any other dye. Use a solution of equal parts white vinegar and water to soak the shirt until saturated. Wring the shirt, and tie it with rubber bands. Apply food coloring in the desired pattern.
To avoid mixing different colors or creating a mess, always apply the food coloring in the correct light order (red, orange, yellow, green, blue, indigo, violet, pink). After applying the food dye, set the colors with heat by microwaving the shirt in a dish for 1 minute on high power then 5 minutes on low power.