Research. Find your own identity by looking at other people's work; identify what aspects of their artwork you are attracted to. Look through magazines, websites and images of graffiti to see what style you like.
Copy small sections of graffiti images you have seen. Get a feel for spraying in these different styles and what you feel most comfortable doing. This exercise teaches you what you like, dislike or are good at or what has the potential to be modified to suit your style.
Experiment with different styles using spray paint on canvas, or scraps of paper. Combine different aspects of the styles you have played around with in Step 2. To customize graffiti, it can be as simple as adapting a pre-existing style to suit you. For example, you might choose to emboss a particular font or curve the letters rather keep them square.
Design your own images. Once you are confident copying and modifying other artist's work, design you own image or font. Customize the image by using a theme, such as science-fiction or retro and adding your own twist. For example, you might choose to use a particular range of colors or add heavy shading to a robot.
Tag your art. A tag is the graffiti artist's signature. If you want to truly customize it, then generate a symbol or tagline that is unique to you. Crews are groups of graffiti artists who tag public spaces illegally, so stay clear of using any of the tags that you have seen in your area. Keep the tag simple so it is easily drawn and recognizable.