Are you already blogging? If not, I would start doing that. Writing short form technical articles and sharing them is like bootcamp for longer form writing. You'll learn the mechanics of organizing thoughts into a linear narrative, the discipline to finish, and you'll get feedback from readers about what works and what doesn't.
I blogged for a few years (and wrote thousands and thousands of comments on Reddit, which was also helpful) before I wrote a book.
I blogged for a few years (and wrote thousands and thousands of comments on Reddit, which was also helpful) before I wrote a book.