Doing this for a friends birthday, Hoping he likes them! :0)
All I did was got a spray bottle, Filled it half full of bleach and half full of water.
Then laid out my stencil (In my case I mostly used twigs, leaves, onions, etc for my "Stencils"
After the bleach set in to a color I liked I rinsed them in water to stop the bleaching process. Then washed them by themselves and dried.
Take a look if ya will :0D