want to choose something without much detail. if the shape is hard to cut out- it probably won't turn out so well on the shirt. print your shape (or draw) onto card stock and cut it out.
make sure you put something between the front and back of the shirt. we just used a rolled up piece of craft paper. put some bleach in a spray bottle. we used 2 parts bleach, 1 part water. place the stencil where you want it on the shirt and start spraying all around, and on top of, the stencil. it will take a while for the bleach to start changing the color of the 