The image above is intended to give a sense of the size and scope of the task you face trying to get your book published. It's immense, but I suppose, too, in its own way it is majestic and honorable.
You are trying to enter the realm of not just published author, but an author with a sustainable career not unlike the literary lights that led you to writing in the first place. For me, it was the short stories of Hemingway, Updike, Baldwin, and Carson McCullers.
Your book proposal is the ticket that you can write for yourself that will get you where you want to go, or at least will help you make the first, most important step. Therefore, you've gotta write a good one.
There are a lot of points, large and small, nuanced and obvious, to writing a good proposal.
BUT there are two key questions your proposal must answer.
1. Is it a good story well-told?
2. Will it sell and make money?
I'll talk more about how to execute on these two questions, but for now you need to think about how you can answer them. If you have two good answers, then you will have a compelling book proposal.