That would depend on the kind of nonfiction you are planning to write. Is it a history? A news article? An advocacy piece? My planning would differ based on the answer.
To be honest, I don't. For me, things like essays seem to come naturally; I just pick a topic and write about it. However, certain subjects I will head to Google or the library and do my homework, especially if it deals with my opinions on scientific research or political events.
I personally open up a document, I organise the chapters and then I organise the topics and then the inclusions for that topic. So it is organised, I am currently writing one about religion and I plan on another two.
I range from no planning, just writing, to intense plans, maps, name lists, character bios, the works. It is all very personal to you
Since most of the nonfiction writing I do involves citing other works, I spend a good chunk of time finding relevant passages and quotes. For me, this is the most tedious part of the writing process.
For the memoirs I'm working on, I've simply planned out the chapters and what they'll be about, namely certain time periods, and which "pivotal" moment they should end on, if there is a pivotal moment to end the chapters with. My memoirs are divided into four "books", each comprised of an account of each year over the four year period that my story takes place, and each book is comprised of ten chapters each.
It depends on how knowledgeable I am on the subject. I usually make an outline and go from there. If I need external research, I'll print/copy and highlight the sources and info, organize them, make citation notes. I'm not the best at multitasking so I will work on each subtopic separately, one or two at a time only, and only when I'm confident with my voice and perspective and/or facts will I start to write. My memoir has been more complicated since I'm working with a nonlinear timeline, have letters and other external things being added, and I never took into account the emotional roller coaster it would be just resurfacing things buried and confronting painful memories. To say it's the most difficult writing I've ever done is an understatement. The catharsis after each 'closed chapter' is almost invigorating, though.
@sunsplash I totally understand what you mean about the "emotional roller coaster." There are some parts of my memoir that I haven't been able to write for almost two years because the memories associated with them are so painful. I would agree that it is the most difficult non-fiction writing I have ever had to deal with.
@Alesia Yes...There are parts I've kept on the back burner for now, too. It really is a different process and way more time consuming than I anticipated. I've also noticed how I've become slightly more objective in areas as time has gone by. The emotion is still there but the parts in healing I can now better visualize outside of just myself.