First, grab a big men's tshirt. I used a size M, and it just covers my bum enough to wear this top with leggings.
Snip the bottom hem off and the arm hems too.
Then cut yourself a neckline. V neck, scoop, its up to you!

Next, cut up the sleeves into tiny strips, ending right at the shoulder join. Make sure they're straight all the way around, so that the strips are even. The thinner the better, but it can be fiddly!

Next, mix 2 teapoons of bleach with a cup of water.
CAUTION: Please do your bleach painting either outside or in a VERY well ventilated room; or expect the mother of all headaches/collapsing!
Mix this up with your paintbrush, and you're ready to go!

Making sure that you lay your fabric on some plastic or layers on newspaper to soak up excess bleach, you can start painting!
I brushed a pattern around the neckline, front and back, with outward strokes of bleach from the inside of the line. And it looked pretty good! Depending on the colour of your shirt, the colour you get from the bleach may differ, but usually its a rusty colour.

IGNORE MY WEIRD EYES :P

Leave the top to dry outside, before machine washing it to get rid of smelly bleach.

ET VOILA!