What most naturally follows?

How to write a fiction book
What do you know?
That is the first question you should always ask yourself because if you don't know about something, how can you write a whole book on it?
Think up an idea.
It isn't as easy as you may think. Every book starts with an idea.
Something that works is imagining a really good beginning, asking questions about it, and sort of working out a plot line to go along with it. Pick up a notebook and pencil, or get out your laptop, and brainstorm.