Decide which alphabet you would like to learn. Graffiti alphabets are varied and depend on the artist who is painting with them. Find an example of the style of lettering you would like to learn and take a few photos of it. This will help you locate the full alphabet online.
Visit a website that lists several graffiti alphabets. Using the photos you have taken, compare the lettering you want with the letters of the website. There will always be some variation due to personal styles, so find the one that matches as closely as possible. (See Resources below.)
Print out a sheet of paper with the full alphabet displayed. Using tracing paper, begin by going over each letter with a pencil. Repeat this on several sheets of paper until you get the feel of how each letter should be written out.
Attempt to write the letters without the use of tracing paper. Use your printed sheet as a guide. Write these out in pencil first so that you can erase any mistakes that you might have made. After you're comfortable with the lettering, use ink over your pencilled letters. Then try writing them using only ink.
Alter the style of the alphabet to fit into your own personal style. As mentioned above, graffiti artists customize their work to stand out from other artists. Add additional lines to your lettering, customize the interior of blocked lettering or reverse the direction of the letters.