Firstly, make your stencil. Any house crest will work. All the houses, plus a few more Harry Potter related things can be found here:
(scroll down a bit to the links)
Put your cardboard inside your tshirt so that no bleach accidentally bleeds through (I used an old cereal box).
Bleach through the stencil using a cotton bud (more control than a paint brush, also no melting - synthetic fibres are not good in bleach). Mine didn't develop clearly because I loaded the bud with too much bleach and it bled, but I'm outlining it so that's ok. Hand wash the top to remove any left over bleach and leave to dry.
One dry, outline your bleach image using the 3D paint. Make sure to iron on the reverse to fix it. I splodged mine in the bottom right corner, so added the 'HP' logo as a cover up. Crafty huh?!
Wear with pride as you try to get through the gates to Hogwarts...