Many many moons ago I got a few things published by APress (Pro ActiveRecord) and O'Reilly (Web Services with Rails) and a few other technical publications.
Throughout working on those projects I missed a ton of deadlines and just really struggled to finish out each project.
I had all the research and knowledge about the things I needed to write about. The "problems" were solved.
I just had to sit down and actually execute, and that's where I really struggled (I've always been a chronic procrastinator — especially once my brain thinks it's solved the problem).
My editor at the time told me to "just write everyday" even if it's just one paragraph...and even if it isn't anything for the book.
Just commit to the habit of writing something every day.
I eventually did and it actually did work!
Some days I would only write a few sentences of something. But more often than not once I sat down to write and worked through the "start" hurdle — I would find a groove and write more. And before I knew it the projects were done.
I've since applied this learning to many other parts of my life (I've got a GitHub streak about to reach 9 years right now and I take the same basic approach to each businesss I help build).
So I know it's said often by lots of people, but I can't stress or repeat it enough: