Hi Sam,
A few suggestions: don’t start with Chapter 1. That’s the hardest chapter to write, because you need a good sense of your book’s direction and content to write it – which won’t happen till you get further into it. Instead, start with a topic you feel really comfortable with and can explain easily. Your plan of writing wherever your interests take you, is a good one!
As to whether to flesh out one section: it’ll probably take at least a chapter’s worth of writing before you have a sense of how things will flow. Work on sketching out at least a chapter or two before moving to close work on one section.
The more you write, the more you’ll find a pattern that works for you.