Wait for it
Like Astinus, I thought of the characters and their goals along with the situation. I then thought of a beginning and end while only thinking a few scenes in the middle. I might be able to plan what I want happened in each chapter, but half the time I end up needing to shift a few scenes I want into the next chapter.
Back when I did Nothing, Everything I actually had the draft of the whole story done in a month and then it would take me anywhere from a couple weeks to a month to get each chapter edited (part of it is I would have a beta or two looking over also). For the story I'm doing now the chapters I'm able to finish within a week and I'm already done with the rough draft of it, but the editing is taking much longer as my betas kept suggesting changes and I kept wanting them to look at the chapter one more time, haha.