Try this process that has guided me through 1700+ freelance articles and 46 books in print. It took me some years to catch on. Nobody told me!
1. Look at your purpose statement (taped to your monitor), read the chapter title, go through your notes, and just start writing.
Start writing, and keep going. You can fix it all later, just get words down.
You never know quite what you're writing until you're writing. :-)