Okay, let me make an example for you
First, you want to make the graffiti and then, you create a new layer and you put the first letter in the first layer, and next (second) layer would be the second letter, and third layer would be third letter, etc.etc. then you fuck around with blending options and ish like that, and you see something you don't like, then you click on that second layer and then, you can freely make any changes to it......
If you make the graffiti all onto 1 layer and you see something you don't like, then, you won't be able to change some of the details on it