Visit a graffiti creation website. A link to one such website, Graffiti Playdo, is in the Resources section.
Familiarize yourself with the palette layout. While the commands are the same across platforms, each website has a different appearance with the controls in different places.
Choose a paint color. You can choose from a variety of grays, white and black, and diverse other colors. You will change colors to mix and match as you go.
Choose a spray area. You can choose a pinpoint stream, a wide circle or anywhere in between. Different spray areas are useful in different situations. For example, a wide spray area is good for bold strokes or for painting large areas at a time. A narrower area is good for detail work.
Choose the paint density. This affects the transparency of the paint. At maximum density, the paint is totally opaque, and none of the underlying colors or wall textures will show through. At lower densities, the paint is more transparent, allowing the colors beneath to show through. This allows for the actual mixing of colors.
Paint on the virtual "wall." Use your mouse pointer to paint your designs, switching color, area and density as you wish.
Experiment with your own designs. The point of graffiti, online or off, is to express yourself with your own artistic ideas.