I write out of order. I skip around within scenes, writing the parts that come easily first. I skip around within chapters, writing the middles, the ends, the beginnings as they come to me. I always write the middles of books first, writing all the scenes that I’m excited about before I make myself go back and write the first chapter.
The reason I do this is based in productivity; if I get bored or stuck with what I’m working on, I can continue to make progress somewhere else until I figure out what to do with the part that slowed me down. I do this all the way through the book, but when I first start drafting, I skip around more than I will later in the process, because I like to start with scenes I’ve already begun when I was doing my freewrites, and then expand them into the emotional heart of the book. If there’s a romantic arc in the book, I’m likely to write all of the key scenes in that arc during this process, because writing the emotional heart of the book helps me to set the tone for the rest of it.
The scenes I write during this process act as structural landmarks around which I can build the rest of the scenes. Once I know what the tone of the emotional keystones of the book are going to be like, I can set the tone of the more plot and action heavy scenes to build up to them. Also, I can write my way into the book doing things I’m excited about, so by the time I get to the stuff I’m avoiding, I’ve got a strong hold on the characters and the voice.
Then, usually after about 6-7k, I’ll run out of steam. There will be no more scenes that really jump out at me left. Then I know it’s time to get that first chapter written.