I posted this on my roleplay blog when I made it last week, but I completely forgot to put it up here! I'm quite proud of it.
This was incredibly simple to make and relatively cheap. What you need is:
a 100% cotton shirt
a spray bottle
undiluted liquid bleach
rubber gloves
a piece of cardboard
a heavy plastic bag or other piece of thick plastic film
a craft knife
a temporary adhesive (I used an Elmer's spray adhesive)
I cut the inverse stencil out of a heavy plastic bag with my trusty x-acto knife. I traced a circle and freehanded the eyes/mouth, but you could trace the whole thing. After you've cut out the stencil, you need to make it tacky so it sticks to the fabric. I used a spray adhesive. Stick it where you want it, and don't forget to position those little eye irises.
After the stencil is placed, make sure you have a piece of cardboard (or in my case, a cutting mat) underneath the first layer of your project. Then, put on your rubber gloves and give the stencil a few sprays of bleach, focusing on the eye area but letting the bleach spread around the stencil.
Let it dry. Once it's dry, you can rinse it out, dry it, and wear it!