Choose a style you'd like to use for the name. Bubble letters are especially popular in graffiti (see the related wikiHow for instructions), but there are plenty of other styles, too. You can have rounded edges or sharp, equally sized letters or some big letters and some small, etc. It's easier to emulate a style, and it's a good idea to have a picture of a graffiti piece in that style to refer to. Once you learn the basics you can develop your own style naturally.