You will need
a clean dark tee shirt - the darker the better
a spray bottle
some rubber bands
a bleach-proof space
Put a little bleach in your spray bottle. Label the bottle so that you don't accidentally mistake it for hair spray some day.
Arrange your tee in your bleach-proof space. I have a plastic mannequin set up on my work table. But you can just lay it flat on some plastic bags.
These two instructables may give you some ideas for layouts:
1) Transforming a boring old black t-shirt using bleach!
2) Bleach Spritz Clothing