If you can’t write, plan. If you can’t plan, write.
I frequently advise nonfiction authors to be planners, not pantsers (that is, seat-of-the-pants writers).
Unfortunately, that doesn’t always work.
True, some people have a conception of the whole book in their heads. This allows them to set down a table of contents and, after doing research, create a fat outline before writing each chapter.
But some people don’t really know how the book is going to come out. They’re just suffused with a passion to share their insights.
Writing will get you unstuck
If you are in this situation, write. Write something that you are passionate about. Write the easiest part of the book. Write a manifesto. Write to persuade a reluctant reader.
Writing can unlock the floodgates. It’s a pretty good way to figure out what you want to say . . . or if you have anything useful to say.
Keep writing until you feel more confident about what you’re supposed to be doing. Once that happens, start planning.
It’s still far more efficient to plan before writing, but sometimes you have to write before planning to get unstuck.
And don’t get too upset if what you wrote doesn’t seem to fit in the book. You wrote it to get yourself going; that passage can do its job even if no actual reader ever sees it.
If you can’t write and you can’t plan, talk
What if you can’t feel comfortable writing and you don’t know yet how to plan your book?
Talk to a friend. Somebody who knows the topic. See if they find what you’re thinking interesting.
Or talk to an imaginary friend. Dictate and record your thoughts.
If that doesn’t work, maybe put the project on a shelf for a while.
Talking can help you plan. Writing can help you plan. Planning can certainly help you write.
All three will help you get going. Good luck!