I DID NOT invent this style of graffiti, just made it easier and put it into an instructable format. As far as I know a graffiti artist named "Moose" invented it [link], and i completely and utterly copied him. :P
If you do something stupid, like write on someone's car with this method, DO NOT BLAME ME. I know there's always these stupid disclaimers, but really, how would you like it if someone wrote on your house or car with bleach?
Anyways, I don't believe this to be illegal, as long as you keep it on cement or concrete, but if you do it on something (Like wood) it might be, so keep it to the street guys. (Just because I don't think it's illegal doesn't mean it is, remember that) On wood, it seems to actually bleach the wood itself, so I think that someone could argue that you damaged their property.
Step 1: Gather Materials
Bleach --the most important part, duh!
An old rag --don't use a new one, it's getting bleached guys!
A cup, or something to hold the bleach
Some nasty cement -- look for cement that seems to have black stuff on it (that works the best)
Gloves --believe me, my hand smelled like bleach for the rest of the day, no matter how many times I tried to wash it
If you want to use something else to write on the cement with the bleach, go for it, I just preffered using a rag, and I happened to have one laying around.