The first paragraph refers to the article and states your reaction to it. The second paragraph expounds on the article and why you agree or disagree. This is where you can inject anecdotes, quotes, statistics and any other information that supports your point. The third paragraph is for wrap-up and to restate your major point. Finally, make sure you know the correct name and spelling of the editor to whom your letter should be sent.
Editors like to be recognized, and accurately.
A misspelled word or typographical error can sometimes cut your chances of being read. See Contacting the News Media for ideas on where to send your letters to the editor.