I start by writing what I already know. I'll describe a character, list actions in the order they happen, whatever. And as I write what I know, I find holes in that knowledge: What happened between this and that? What caused my character to be so bitter? Etc. Then I start brainstorming possibilities: Maybe this happened. No, that doesn't work because of whatever reason. It could be one of these other options... etc. etc.

Cheers,
Michelle