Doodle in your sketchbook, brainstorming the types of characters that appeal to you. Draw more cartoonish characters, or more realistic characters. You can even take a character that you like from television or a comic book. There are no strict rules for what makes a good character in graffiti, so develop what you like.
Practice drawing your character in a few generic poses in your sketchbook. Master the proportions and drawing the character quickly. The ability to draw the character quickly is key for hand-drawn characters, but not as necessary for characters you plan to stencil.
Develop a color scheme for your character. Try out different combinations of color with your markers to get a sense of what will look good when painted full size on the wall. Decide whether your character will always be colored the same way. If your character is meant to deliver a message, decide how color will affect the work's meaning.
Compose, in your sketchbook, the piece that will eventually go up on the wall. Carefully mark out the colors and the details. Decide if the piece will include simply the character, or also text or tagging.
Spray the piece on the wall with spray paint just as it is drawn in your sketchbook. Step back from the wall and admire your work.