The steps to writing a proposal are outlined below. Be advised that these steps are general, and may require some tailoring to your specific book or publisher's requirements. The first step is to have a great idea for a book. Some good ideas can be turned into articles, but are too short to be written in book form. Make sure your idea can carry itself throughout a long manuscript.
Some writers work with agents who sell their proposals to publishers.
Others work directly with publishers. If you are seeking an agent, find one who specializes in the type of book you are writing. For example, if you are writing a gardening book, you will not want an agent who specializes in murder mysteries. If you want to work directly with a publisher, writers' reference books will serve as a guide. Again, the Internet is another resource.