Locate an appropriate publisher that publishes books like yours. Do this by finding similar books in a bookstore or library and visiting the publisher's website to read their writer's guidelines.
Write a one-page letter to introduce yourself and your book. Your professional biography should be short, no more than one paragraph, but be sure to explain what makes you qualified to write this book.
Include a one or two paragraph summary of your book including the genre and page count. Remember the query letter should be no longer than one page total.
Check the correct name, spelling and gender of the publisher's acquisitions editor. This seems an obvious step, but there's no faster way to have your query letter trashed than to get the editor's name wrong.
Follow the publisher's guidelines and send the query by their preferred mail method. If they want snail mail don't send an email, or vice versa.