Here’s a guide to the structure your proposal should take:
Paragraph 1: This is your elevator pitch - you need a few lines to sell your book. This needs to be concise and to the point. Here is a good starting point to learn more about elevator pitches in general.
Paragraph 2: This is a more detailed outline of the book. It should consist of a guide to the book's structure and the basic narrative thread of the story. It should also include a word count.
Paragraph 3: Here you need to define the genre of your book, detail your potential market, suggest similar titles, whilst also giving any details for future title.
Rest: This is an extract of your work - fifty double spaced pages is standard. Make sure you pick the FIRST fifty pages. The reader needs to get a feel for the book.
NOTE:
An updates version can be found at the BubbleCow website.