If you want to work directly with a publisher, writers' reference books will serve as a guide. Again, the Internet is another resource. And be sure that the publisher's specialty matches the topic of your book. An agent or publisher may have a preferred template for the proposal. Research this via the Internet or by contacting the publisher or agent.
Some publishers place their templates on their web pages.
Or maybe they prefer a disk. Your proposal must include information on existing or proposed competitive books. Do an Internet or bookstore search for similar books. You must make sure that your book can sell in the midst of competition. Information about you.