How do you write?

I've never really been able to explain how I get ideas for books. I know sometimes that I take ideas from things I have seen and experienced, but mostly I just sit down and the words pour out from nowhere.

So, when I read Barbara Stoney's biography of Enid Blyton a few weeks ago, I was very pleased to discover that Blyton wrote how I write. She terms it her "under-mind":

... My hands go down on my typewriter keys and I begin. The first sentence comes straight into my mind, I don't have to think of it. I don't have to think of anything. The story is enacted in my mind's eye almost as if I had a private cinema screen there ... I don't know what anyone is going to say or do. I don't know what is going to happen. I am in the happy position of being able to write a story and read it for the first time, at one and the same moment ...

I don't pretend to understand all this. To write book after book without knowing what is going to be said or done sounds silly - and yet it happens. Sometimes a character makes a joke, a really funny one, that makes me laugh as I type it on my paper - and I think, "Well, I couldn't have thought of that myself in a hundred years!" And then I think, "Well, who did think of it then?"

That pretty muchs sums up my writing process, but how do you write? Do you plan? Or do you have the under-mind?! -x-