Bleach is fun!
And if you don't think so you're not using it right.
It's all very well putting it down the toilet, but it's much more fun to paint with it.
I think henna designs particularly lend themselves to this technique as they're basically line drawings, which makes them relatively simple to replicate.
Plus, the colour the bleach reveals also reminds me of henna staining.
But anything goes really!
1 - Use a appropriately sized piece of study cardboard to both stretch the design area flat, and also protect the back of the t-shirt from the bleach you'll apply to the front.
2 - Sketch out your design in chalk or some other visible yet washable marker. Once the bleach has started working you won't be able to stop it, so you want to get your design right first time.
3 - Using an old paint brush (the bleach won't destroy your paint brush, but it won't do it much good either, so use an old one) and a glass of bleach paint over your design.
Apply more layers of bleach to areas you want to be a lighter shade.
4 - Once you're done, wait a few minutes then wash the t-shirt as usual.
I would wash it by itself this first time, just to make sure nothing else gets accidentally bleached.
Obviously, don't get bleach on your skin, clothes, surfaces etc.
It's a good idea to protect all of those things.
Do you want more fun with bleach? There is also a great tutorial for bleach tie-dying over at Honestly WTF.