Legal letters serve many purposes. For instance, they can remedy a problem between two parties in lieu of filing a lawsuit, or they can play a role in contract negotiations and other legal matters. If you're writing a legal letter, keep it professional and write precisely and concisely so that you leave no doubt in the reader's mind about your purpose for writing the letter. To do this effectively requires some time and effort.
TL;DR (Too Long; Didn't Read)
Legal letters should be in proper letter format. Use concise language to set forth everything you want from the other party or everything you want the other party to know.
Composing the Introduction
Type the full name and address of the party you're addressing at the top of the letter and include the date you plan to send it. Use a standard font like Times New Roman and black ink to make sure your letter is legible. Address the other party by name and use a courtesy title, Mr., Mrs. or Ms. If your letter is to a company or you don't know who will read it, begin the letter with "To Whom it May Concern" or "Dear Sir or Madam."
Citing Enclosures and the Reason for the Letter
Cite any enclosures to your letter. Mention these first so that the other party can verify that you sent them and look them over before continuing with the letter. An example would be starting your letter with "Enclosed is a copy of our contractual agreement dated..."
State why you're writing the letter and give the specifics of your case. Include the names of any related people or companies and the exact dates of any interactions or incidents. The more detail you can include, the better.
Giving Specifics of the Law and Your Goals
Cite any laws that apply to your letter, if applicable, and explain why the law applies to your writing the letter.
State what you want in the letter, whether it's a payment or an action of some sort from the other party. Give a specific time frame (two weeks or 10 business days is standard) for the letter to be responded to before you take further action.
Proofread and Send
Proofread your letter for errors and to keep it concise. Eschew passive verbs. Use the active voice instead. Edit out any words or phrases that aren't necessary. After you proofread it, sign and date the letter at the bottom. Send it the same day you write it.