Keep practicing. If your graffito turned out perfectly, congratulations. Now you can move on to different words and effects. If not, keep trying. Mast...
Outline the letters you have written to approximate the style you're going for. Again, use pencil and write lightly. You'll likely make plenty of little mistakes, but that's what erasers are for. It will take time and patience to get the letters just right. Remember:
Make the original words you wrote your guide,
not your prison.
Don't be afraid to alter the letters
beyond all recognition.