Draw several ideas for your tag in your sketchbook. If this is your first time coming up with a tag, this may take several tries. Experiment with unusual spellings and letter shapes that help enhance the visual appeal of the tag as a whole. In tagging, legibility isn't as important as good looks.
Practice your tag several times in marker on your sketch pad until you've mastered the strokes and can execute them quickly.
Pick out a new bright color of marker and draw a large version of your tag in bold lines on a blank sheet in your sketchbook. Outline the tag with another color that either contrasts or is darker than the first. This is a sketch of a "throw up." Try different combinations of colors until you find something you really like.
Purchase some cans of spray paint in the colors that you've decided on in your sketchbook. You can get them either at a home improvement store, a crafts store, or online.
Spray the basic tag in wide letterings on the wall or other surface. Apply enough paint to cover the surface opaquely, which may require multiple coats.
Spray your other color outlining the first. Vary the width of the outline to cause the image to pop off the wall. If you make mistakes, go over them with the first color, and then reapply the outline.
Step away from the wall to check the work, and fix any mistakes or awkward points.