Select a subject for your sketch. Many people choose to graffiti some form of writing, while others choose to graffiti objects such as animals, symbols and people. Sketch out your idea; you will have time to go back and perfect it later. Your piece should have cartoon characteristics about it. Graffiti is not meant to look lifelike.
Keep all of your outlines chunky. Since graffiti is created on a large scale, such as on buildings and highway overpasses, everything needs to be big. Remeber to sketch everything wide and thick. The thicker your outlines are, the more your piece will stand out. Keep in mind that you are working on a smaller scale. Use a thicker marker to get the feel of exactly how the outlines should look.
Shade and color in your subject entirely. Once you have finished with that, work on a simple yet bold background for the piece. Backgrounds can include solid colors, smaller symbols or a cloud surrounding the piece.
Experiment with the colors you want on your piece. Too many colors will make things look too busy, but only one or two colors on a complex piece could make the subject difficult to view. Choose colors that will stand out from a distance. For instance, red beside orange may look good up close, but they may be indistinguishable from a distance.
Practice sketching words as well. Words are a huge part of graffiti art, and should be sketched wide and thick just like subjects so they will stand out. Overlap letters to make the word look three-imensional from far away. Choose a font that will be easy to read, but one that is not boring. Use graph paper when sketching to make the letters proportional. Add objects to the background when you are happy with the word.