I've found that it's been different with each book. The first one I wrote, I started with a scene and went from there with no clue how it would turn out. It was
really fun. The second one took up from where the first one ended and I didn't know how that would turn out either.
I decided the third one would be the last of a trilogy. I started where the second left off and an outline of what I wanted to happen. The journey's gone a bit off the track here and there, but it's pretty much following where I wanted to go.
The fourth book is actually a return to my first attempt at a book five years ago. I wrote 40,000 words and realized it had serious flaws. I've come up with new ideas for it, but it's going to start at the same place. That will be a trilogy too, but I don't know where books five and six are going to be yet.
I've also got the first two chapters of book seven written. They came to me while writing book three and I decided to get them down before they disappeared.
I also have a rough idea of how book ten is going to go. That's a little way off though.