For me, it all starts with an idea. Once the idea is formed, I start designing characters and events to flesh out that idea. If I'm working on something else at the moment, I write those things down so I can get to them when my current projects are finished.
Sometimes, though, I'll sneak in a few words here and there on the manuscript, if the idea keeps nagging at me. The worst I've ever done (and possibly the best) was taking two days away from my current manuscript and writing fifteen thousand words on the new one, because it wouldn't leave me alone. That book was Blood Double, the beginning of the God Wars series.