They are under no obligation to run a letter. Don't expect a letter to be published immediately. A letter to a large city daily, for example, could take 2 or more weeks to be published after it is received. If you are responding to a specific report or issue, send your letter quickly while the topic is still current. The address and fax number are provided in the publication.
Shorter letters have a better chance of being published.
If a letter is too long, it may be edited down by someone who probably doesn't understand the issue. Use logical or chronological order. Keep the tone objective and professional. Don't be offensive or make personal attacks. Focus on the information not the person.