First, there's an idea. Just a little spark. A moment of "what if?"
For many people it ends there, because of demands of the outside world, loss of interest, fear. It's daunting to start writing. There you are, sitting with your blank paper or empty screen, trying to make something magical happen. You want to write the perfect book. And as long as you think that way, you'll never get started. Writing is messy, cluttered, sometimes random. You have to get this mess into a computer or a ream of paper so that you can begin the process of revising. This is where the real work is, where you take that mess that you made and turn it into a book. It's not as fun as writing, but it's just as important. Or even more important.
I make a lot of notes on a separate Word document when I write. It's full of good sentences that don't have a home yet, a few words to denote a scene I want to add, random ideas which may or may not get used. It's messy, and I have to scroll through it to find what I'm looking for, but it's a place to put what's in my head somewhere until I can attend to it fully. I don't worry about spelling, punctuation, or grammar in this document. If I didn't make this document, I would have a thousand post-it notes stuck all over the table, in a certain order, and color-coded. Which sounds a little like the opposite of my catch-all "Ideas" file. And I would probably spend more time organizing the post-it notes than writing, frankly.