Begin your letter by entering the date followed by a blank line, according to standard business letter format.
Put the editor's name and title (or agent's name) on the next line. Be sure the name is spelled correctly.
Type in the publishing house or literary agency, street address, city, state and ZIP code on the following lines.
Skip a line and put in your salutation--"Dear Mr. Banks:" as an example. If you don't know whether the editor is a male or female, do more research.
Put the reason why you have chosen this particular editor or agent in the first paragraph. It could be that you heard him speak, read an interview from him, read his blog or liked books he has edited. Be specific--for example, "I heard you speak at the conference in Denver last spring. I enjoyed how you showed pictures of your office and were candid about the problems you see with many submissions." If this is a requested submission, say so.
Write a blurb about your book. Think jacket flap or back cover copy. What would yours say? Write as compellingly as possible. For fiction, share your main character's name and hint at her problem. This does not tell the complete story, nor is it a line-by-line listing of what happens in the plot. For nonfiction, include the purpose of the book. Finish the paragraph with details about your manuscript such as title, genre, word count and any additional materials. For a magazine piece, mention the rights you are offering.
Share your writing credits, if you have any. If not, you might want to mention appropriate education--for example, a degree in journalism, membership in professional writing organizations, awards and anything that makes you especially suited to have written the book. If you are a child psychologist, and your book is on child psychology, this gives you credentials. The letter is not a complete resume, but it is a brief summary.
Tell whether you are sending an exclusive or simultaneous submission. If you've included a self-addressed stamped envelope for the manuscript or for a reply, say so. Always include appropriate postage on any return envelope.
Use a businesslike closing, such as "Sincerely yours," followed by three blank lines and your typed name. This leaves room for you to sign your name once the letter is printed.
Include your full mailing address, either below your name or on your letterhead. Follow the format you used for the addressee.
Include your phone number and an email address, each on a separate line.
List your writing website or a website related to your topic, if you have one.
Add another blank line, then type "Enclosure" or "Enclosures," followed by a colon, then type what you've enclosed in the letter--for example, manuscript, SASE or photos. Print the letter, and send it.