How do you write a series of books?

Write the first one, end it, and then see where you want to go from there. Sometimes a story suggests its own longer arc, and sometimes characters will tell you that there's more to their story than what you've told. It just depends--I wrote my first one as a standalone, but I'm working on a sequel to it right now that's not a "continuation" so much as a "new adventure (now with more pain and suffering for all involved!)." The first one might see print; the second one I'm...not so sure about.