First, you get the idea. It may germinate for a long time or it just pops into your head. And then you work out a structure. And when you feel confident enough, you start to write. And you have to allow yourself the liberty of writing poorly. You have to get the bulk of it done, and then you start to refine it. You have to put down less than marvelous material just to keep going to whatever you think the end is going to be—which may be something else altogether by the time you get there.