Generally, the best advice I’ve seen in a while is to – basically – start high level and add detail systematically.
For example:
Write an overarching goal or mission statement. What’s the gist? From there, list the chapters that go to support that goal. Then, for each chapter, dot-point out the key points. Those are you subheads. Then dot-point the key arguments for each subhead. Those are your paragraphs. Fill ‘em out.
Basically, you’re adding depth layer by layer. If you get stuck on anything, just slap filler text in there (e.g. whenever I sit down to write an article, I just use some keywords for the title and then write [INTRO]. Introductions are hard to write and, if I start right away, I’ll just get stuck.)
I can’t remember any non-fiction specific guides, unfortunately. My favourite overall story advice is Film Crit Hulk’s Screenwriting 101 (it’s relevant to more than just screenwriting).