I second Zinsser, great book.
Mainly: write, then edit, re-edit etc. Keep going until nothing can be improved. In Clean Code it says, no-one can write clean code - you write dirty code then clean it. Every first draft of a novel sucks. Writers do 50 drafts and it starts looking OK. Every word carefully weighed dozens of times. No-one can write perfect prose in the first draft. Write a terrible first draft, of a chapter or part, then you have raw material to edit. Don't judge the first draft for quality - it's supposed to be bad.
Also, I've found that you often learn as much about the subject while doing that as you knew before! It's a great way to learn. Questions arise as you're doing it - make a temporary list of them at the end, most will answer themselves or can be answered in minutes or hours. If any are left, leave as Exercises for the reader :-)
p.s. Some people say it's way better doing it on paper. When "cut and paste" was with scissors and glue, you could cut everything up and move the parts around before starting to paste, as many great writers have done - on a computer screen you can only move one section at a time. Cut, paste, cut paste.