Most of us have had the experience of explaining something in detail only to have the "explainee" award us a blank look and a huh? I used to teach adult education classes in another life. Actually, I taught the beginner-beginner computer course. We called it Computer Boot Camp and the course included things like using a mouse and turning the computer on and off.
I'll never forget the woman who arrived the first night of my class, buttonholed me right inside the door and announced, "I may have to take this class to keep my job, but nobody can make me touch little furry animals!" I might add that when she received her certificate at the end of the class, she and her mouse were best friends.
Explanations and teaching are close kin. Both require the ability to break down knowledge in its smallest component parts and then share that knowledge in such a way that the receiving party can reassemble the parts into a harmonious whole. The only way for the teacher/sharer to do that is to have the ability to remember the time "before."
Yes, I know you wonder what I'm babbling about. So... something simple. How to change a diaper. After the first million or so, you can do it in your sleep with both eyes closed and one hand tied behind your back. But if you're going to pass on that knowledge, you have to remember back to the first time you changed a diaper. Remember how nervous you were? How you were sure the kid was gonna break?
That's what you have to do to effectivly pass on that knowledge. Make no assumptions about what your target might know. Simply start at the beginning.
Writing is much the same way. In every book I write, I find a place where I forgot that my reader doesn't live in my head. Then I have to go back and clear that up. Recently, I had Final Line Edits on a new release. My editor pointed out a fairly lengthy passage and said, "They're actions make no sense. Explain."
In trying to justify what I'd written, I realized that the editor (and reader) had no idea what was happening because I'd failed to start at the beginning. Instead of beginning my instructions with place the baby on a firm surface... I was racing ahead to wipe the baby's bottom. And my hapless student was left with a diaperless baby.
Planning a book is much like building a lesson plan... step one, step two, step three. To avoid confusion, start at the very beginning.