Step 1: Gather supplies
craft stick (or disposable wooden chopstick)
plastic bags or other dropcloth to protect your surface
cardboard & tape
Step 2: Build shirt insert
Cut the cardboard & tape pieces together to fit perfectly into your shirt. Take your time & get it right and it will make a big difference in the finished appearance of your shirt. (random paint spots seeped through to the back are not pretty)
Step 3: Choose colors & prep paint
Because you're painting with a sponge, you need less paint spread over a larger surface area. I achieved this by putting a small amount of paint on the wax paper & then folding the wax paper over & smooshing it. Wahlah; your very own mini rorschach!
Also, if this is your first time using this sponge (as it was for me) you may want to grab some scrap paper & just test it out & get a feel for the look you want.