I have been asked this question several times. Every writer is different but I found what worked for me with this book, somewhat surprisingly, was a hybrid approach. It went something like this.
1) Write out the main ideas for the storyline.
2) Loosely work out the arc of the story.
3) Create the main characters – about five really, to begin with.
4) Systematically plot out every chapter, start to finish, including a short chapter summary, the scene (where), the goal of the chapter, the characters involved, and how the chapter fed into the overall story-line.
5) Start writing (after feeling good about finishing #4.
6) Except here’s where it got surprising and interesting. At about Chapter 3 I started to deviate from my detailed outline. As I got more into the characters themselves, I found they started to take over where the story would go next. At first it was a bit frustrating and I thought I’d be going backwards by not sticking to ‘the plan’. Then after a while I let go of that and just went with the flow.
7) Sure, there were plenty of chapters moved around as well, but the nice thing was that I was always thinking about the plot, and I think that helped me write the chapters. Not knowing exactly what would happen through the book kind of kept me alive and engaged with the writing – and there were very few days over the course of several months where I wasn’t thinking about the book and the characters.
8) Created a soundtrack. Sounds odd, but there were a number of songs that seemed super relevant to the storyline and I played those songs A LOT.
9) For the second book in this series I’m doing something different. Instead of the detailed outline (I still think it was a good idea as it helped to shape my thoughts), I now have the storyline and main chapters mapped out with sticky notes on a wall hanging. That seems to be working pretty well.
Of course, the extent of how well it worked for the first book in the series is really over to the readers!