It starts with a quote, a scene, or an idea. Usually, I will text or email myself the idea if I am out in public. But most of the time, I will jot it down in my notebook or a blank Google doc. I am your normal author. I can't plan out the whole book and then go in and fill in the details. So, my process is a little barbaric. I usually jot down those thoughts or ideas at the end of the page, and as I write, I will see where I can fit those ideas. I plan the important stuff, of course, first. The characters and a major plot, so I have an idea of where I want the story to go, but I won't know it until I write. Then, as I write, sometimes things change and move around. Characters are born, and some are changed personality-wise. Sometimes more than one plot is used, and that's okay too. If I can't think of anything within a chapter, I skip it and move on. I find it easier to write certain parts first to get an idea to set up the scene. Sometimes it helps to take a break, watch tv or read a book. If I can't think of anything or don't have any motivation to write, these things influence me to get my job done.
I had this issue too. Believe it or not! It is so easy to get distracted. My problem was that I was coming up with too many ideas and too fast. I wasn't giving myself time to process one idea at a time, so I bounced from idea to idea. Writing a few chapters and then off to another idea leaving one behind. This resulted in 34 plus unfinished works and over 70 chapters that had an unfinished story. So, how did I break out of this? I had to shut my brain off. It's not easy, but when I formed an idea, I stuck with it. Then, I went through some of those unfished stories and chapters and began to pull together one big awesome story. This eliminated some of my unfinished work, and I created my first novel. So, the point is, sometimes all of those "boring" ideas can be used as step-stones in one of your chapters. It can be a plot or even just a filler.
I started writing HOTN in September/October, following another story I didn't finish. I finished writing HOTN at the end of January.