Practice your letterforms with marker on paper. Write out the whole alphabet, not just your tag name. For a challenge, randomly choose words from the dictionary and use your handstyle to write them out.
Choose a can of spray paint and fit it with a new cap. Skinny caps are most commonly used for tagging, though you can eventually use fat caps (good for creating flares) as you gain better can control.
Hold the can three or four inches from the surface you're tagging, with the nozzle pointed straight ahead.
Use quick, single strokes to recreate the letterforms that you practiced on paper. Focus on one letter at a time. Don't worry about connecting letters at first.
Keep practicing until you achieve clean lines on all of your letters.
Start connecting the letters in your tag until the characters become a single entity.
Embellish letters with crooked or flared tails, but only use these devices if it enhances the tag.