What you need

Your garment.
Household bleach.
Rubber gloves or similar to protect your hands.
erm… THAT’S IT!
I used an old black t-shirt I found lying around (see the second photo). I’d expect you get an even better result on brightly coloured garments. I really can’t wait to do some more.
What you need to do
Wet it: Take your garment and get it wet through in the bath or sink, then ring some of the water out.
Bleach it: Take your bleach and squirt it liberally on random areas of your t-shirt, leaving some bits uncovered. Rub the bleach in slightly, scrunch the t-shirt up a bit, squirt on some more bleach if you like.
Leave it: Leave to soak in but keep checking the colour every minute or so until you are happy with it. (I left mine for 10 min )
Rinse it: When happy with the colour, rinse off the bleach and then throw your garment in the washing machine for a cycle.