I'm honoured that you ask me for advice. So I kind of read other books to see how they did it. The book you read isn't my first my first book sucked and I never really posted it on here I got some tips from a real book writer. I can give you the tips she gave me if you want. She's a professional and I'm just a starter. I'm not that good yet I'm still finding ways to evolve.
@liliVa3 and @IanoftheG make sure there is an objective for the characters to achieve by the end of the book it really helps. And yes I write books and fanfictions
You said you already have some drafts so why don't you try writing it down on paper. And maybe try experimenting with it to see how you think you should start. Every writer is different and writes everything different. Experimenting normally works like you do while drawing.