Writers

I sometimes wonder how serious writers write the stuff they do. Like the auther of Harry Potter. Did she just sit down one day and start writing what ever came to mind and then sort it all out later? Or do they think of a story in their head first, outline it all, and then put in the filling? The process is interesting to me. I'd like to write a book someday, but have no clue where to start! Ah well, something to dig into later...