1. Described the goal of the book, who the intended readers are, who the closest competitors (i.e., the best on the market) are, and how your book compares with those,
2. Prepared a detailed list of contents, covering the whole proposed book, and
3. For each proposed chapter described the main items.
The more details you provide, the easier it is for the editor to make a good judgement on the market for it.
I have done this four times, for different styles of books.