Write the word or message on a piece of paper. Space out the letters so you have room to expand them. While most graffiti is done using capital letters, you can use lower case if you desire.
Take each letter and decide if you want to angle it. The writing doesn't have to have the letters running straight across, like they sat on a line. They can be tilted. They can all tilt the same direction or tilt in a variety of directions. Experiment.
Choose a style for your letters. Making the letters bubbly and rounded is popular. So is making them sharp and angular. Your style can have jagged edges, pointy ends, or round, bulbous ones. Once you have a style in mind, draw around each of the letters, giving them thickness while incorporating the style.
Rework the outlined letters. Try varying the thickness of the lines in places. Allow some letters to overlap with others. Perhaps some letters are larger than others. Here is where you put in some artistic elements.
Add on arrows or star bursts or bubbles--or just have swirls coming off the words. Decorate your message with visuals.
Using colored pencils, plan out a coloring scheme for your graffiti message. You can have multiple colors in each letter, or do different letters in different colors. You might have patterns or images drawn within the letters. Use your imagination.