Look up and become familiar with different types of graffiti; several styles include wavy, jagged and bubble alphabet styles. Choose one of these styles to practice. Use a piece of paper, perhaps heavy stock paper found in art stores and a hard lead pencil, preferably a No 4.
Draw two or three large alphabet letters lightly, with lots of space in between letters. Draw your chosen graffiti style over these letters. Add a thick layer to the letters, rounding out the corners for the bubble style. For the ragged style, use sharper angles. Make wavy lines for the wavy styles.
Overdraw your letters, varying the space between letters. Exaggerate every aspect of each letter. Experiment and add your own personal flair.
Add shading with a soft lead pencil to add dimension to the letters. Try a 9B pencil, which has a very soft lead. Add a 3D effect to the letters by shading the edges, making sure you edge the same side of each letter for consistency.
Use color pastels or colored pencils within the letters. Use bright bold or neon colors for graffiti. Feel free to mix colors in each letter and blend.