Just keep it simple.
Be sure to give them your name and SS# and then just state that you have noticed errors on your report (give report # if possible), and then the reason you are disputing.
Look carefully at EVERYTHING and if you see any errors, list that reason (was never late, wrong addy - never lived there, balance incorrect, ect.) If you don't have a specific reason, just say "I have no knowledge or records of this acct". Don't say "NOT MINE" unless it is truly not yours or it will probably put out a fraud alert on your reports.
Then at the bottom of the letter, be sure to say something like......please investigate these items and send me an updated report once the investigation is over. Some of them won't send you a new report if you don't specify that you want one.
Also, handwrite these if possible. If not, type in purple or teal italic font......that way the computers can't scan them and an actual human has to open it. Just gives you a better chance for deletion.
That's pretty much it.

Hope that helps.
Good luck!