I want to write a book, but the thought of such a large scale project overwhelms me. How do authors break down a book idea into individual accomplishable steps?

There are a thousand answers to this question. This is just one of them. If you do not like it please feel free to ignore it. Every author thinking of writing a first book is overwhelmed. So do not think of writing a book. Think of writing a short story. Write it. A short story; five, ten pages, even two pages will do. Then sit down, your short story complete, and think about it. I imagine you sitting there, its dark, the curtains drawn, the room lit only by the glow of your monitor or laptop screen. You have been sitting there for a while thinking about your story. Has something changed? have you thought to add a description, or more about the characters, or how it could be different? What would have helped? what did not work.

Sit down again and rewrite your short story with all the ideas that have entered your head as you just sat there in the twilight gloom of your computer screen. It’s a little longer but nowhere near a novel.

Sit down again, this time with a glass of wine and imagine where that story could go, what it may do, the ways in which it could change. This time, do not rewrite, but just add to it. develop the characters, the scene, the place.

Another glass of wine and ignore the story. Just think of the people in it. Who are they? try and think of only one at a time, who are they where did they come from, how were they brought up, raised, money, house, friends, back story. And you will see where I am going with this. Your two page short story can easily become a three hundred page novel. It just takes a great deal of time effort and a good idea.