Start with why. If you don’t know the purpose of your book before you write it, no one but your mother will read it. 

Many people I’ve worked with think they know what their book is and why they are writing it, but it’s only when I pose questions they connect the dots. 

Here are some examples of questions I ask my authors: 

1. Why are you writing this book? (This question gets to the core of your book’s purpose, and can help set reasonable expectations for the success of the book. If you are writing your first book because you want to be a NY Times bestseller, then you may need to rethink your goals). 
2. What inspired you to write this book?
3. You have the completed book in your hands, who are you going to give it to and why? (This question helps define your why, but also closely relates to your target audience, which is the next step.)
4. What is the primary message or theme you want to convey through your writing?
5. What is unique about your book idea?
6. What emotions or feelings do you want your readers to experience while reading your book, and why are these important?
7. What do you want readers to remember most about your book, and why do you believe it is important?
8. What is the last call to action in your book?

If you have a strong why to lead you, it will be much easier to finish your draft. 

Answer these questions as you create your outline. Clearly define the purpose of your book by starting with why, and your book will write itself.