Every nonfiction author has his or her own style and approach to writing. It does not really matter how you write as long as you DO write on a regular basis. By regular, I suggest daily. Someone asked me not long ago how to go about writing a nonfiction book. My answer was simple: “START!” As I pointed out to her; if you write one page a day for one year, you have the makings of a book approximately 200 pages long, based on formatting and what you add at the front and end. I also reminded her what the Chinese philosopher, Lao Tzu (570-490 B.C.), said: “The journey of 1,000 miles begins with a single step.”
Personally, I believe that each writer must discover a rhythm or pace that will sustain them and that they are comfortable maintaining. Like race horses and runners, some are built for speed and others for endurance. If you are a high energy person who likes to get up and start writing immediately, knock out a goal for the day, and then move on to other things; perhaps getting up early in the morning will work for you, as it does for me. On the other hand, if you like to plod along, write a bit, take a break and then get back to it throughout the day; perhaps this becomes your writing pattern. Experiment and see what makes you feel most productive. It often comes down to what circadian rhythm (internal 24-hour physiological clock) you prefer. Some writers are morning people while other are evening people. You likely know this about yourself already, so maximize your high time of the da and write then, if possible.
Another thing that I have figured out about writing is that if your heart is not into the writing, you will likely avoid it and encounter writers block. If you have something so your mind or really are not in the mood to write for whatever reason, perhaps you should take a day off. Give your brain a rest and gather some thoughts about what you will write, then comeback to it the next day.
No matter what approach you take to writing, enjoy it. Anything less will lead to frustration and an inferior result.