Steps to follow:
Decide why you are writing the letter. Are you responding to another letter? Do you want to call readers to action? Is your goal to educate others? These are all good reasons to write a letter.
Know the publication's guidelines. For instance, the Daily Gazette and Observer-Dispatch request letters to be no longer than 250 words and limits writers to one letter a month. You can usually find the publication's letters to the editor guidelines on the editorial page or on their web site. Keep it brief, the best letters are short and limited to a single point otherwise letters are likely to be condensed in the editing process which may cause the letter to be altered. Use a business letter format. Make sure you sign your name and even if you email the letter be sure to still include your address and phone number.
Determine one to three messages you want to express in your letter. Jot down your messages before you write. Avoid rambling and keep in mind most newspapers are written for readers at the fifth grade level. If you are responding to another letter, avoid repeating negatives by touching on the point and going with your own positive message.
Most publications now accept signed letters by facsimile and some will take letters by electronic mail (with a phone number to verify authorship).
Congratulations - now you have the tools to write a great letter to the editor!