This is a question that plagues most writers dreams and has non-writers scratching their heads.
When you have pitched your book premise and an agent or editor requests to "preview" the rest of your book idea (yeah, we're not even at the actual book chapters yet) then you submit a formal book proposal.
Below is an abridged version of an article from The Bindery called "The Anatomy of a Compelling Book Proposal" by Alexander Field. A lot of good advice too! (https://www.thebinderyagency.com/blog/theanatomyofacompellingbookproposal)
1. Book Title & Subtitle
List the title for your book. Include an evocative, intriguing subtitle for nonfiction, and a series name (if there is one) for fiction. If you have alternate title ideas, it is okay to list those in the book proposal for possible publisher consideration.
2. Concept Snapshot
This is what we call an elevator pitch that is 1-2 lines in length to try and grab the reader’s attention and keep him or her reading.
3. Author Biography & Information
The "about the author" section to address: Does this person have the experience and credibility to write on this topic? Be sure to include any experience, background, or information that will give your proposal or your story a fuller picture. Include any previous writing or publishing experience. If you have written and published a book before, include any and all relevant sales information—or at least, an explanation about why that information might not be relevant.
4. Summary / Overview of Premise Take a few paragraphs, or even a page or two, to give your reader a sample taste test of your book. The summary should build interest by teasing out the content of the book, while holding back just enough.
5. Book Details
There are key pieces of information that help the editor create a business scenario for the publisher, including the word count of the book, the timeline required to complete the manuscript (if it’s not already complete) and any other special features you desire to be in the final book, such as an index, photo insert, map, or recipes etc.
6. Marketing & Publicity Opportunities (Platform)
"Platform is the ability to influence others to buy your [your book]." Platform can include a long list of things such as: a blog, podcast, TV show, radio show, business, social media following, nonprofit organization, Youtube.com channel, network, freelance writing, and more. But here’s a rule: Don’t force platform. [Be truthful and don't exaggerate.]
7. Competitive Overview
List several important books that are similar to the book you’re writing, and then describe briefly how your book is different or unique. Note: Only list books that appear to have been somewhat successful in the marketplace. If you list competitive books that sold poorly, the publisher might start thinking your book will sell poorly too.
8. Key Connections, Endorsements, or Partnerships
Who is likely to support your book? What influential people do you know whom might rave about your book? Are there businesses or organizations that might support your book due to a common cause or shared mission? What friends, family members, or colleagues would be happy to post about your book in their social feed? Could you invite them to launch team on a social site such as a private Facebook group? List any key opportunities in the book proposal.
9. Table of Contents
Include an outline for your book. For fiction, chapter summaries can be helpful to get an overview of the plot of the book up front before your future publisher reads any of your sample writing.
10. Sample Writing [or finally! the actually chapter samples of the book!]
Including sample writing will give your publisher a taste or your writing style. Include the first two [to three] chapters. [Usually the first 50 pages] [DO NOT send the entire manuscript.]
Complete the proposal as stated about. PROOF READ it. Get someone else to proof read it. Then send it off [most want an electronic document as in a doc or pdf, but be sure to check the SUBMISSION GUIDELINES from the agent or publisher website.] Then you wait...But keep writing and revising your manuscript while you wait. :)