In the back of The Shepherd's Crown, I found an interesting bit about who Pratchett wrote, so I've been exploring this a little further today and found this interesting interview. In case you think he's a total pantser, in other interviews he talks about how those first 15-20,000 words are the pivotal scenes in the story and that he then goes back and fills in. Oh, and as someone who loves to work with wood, the Carpentry School of Writing works for me.
Q: I'd like to talk a bit about the practical side of being a writer. You've said you are from the Carpentry School of Writing. And you think it's very important that writers work on their craft. Could you expand on that a bit?
Terry Pratchett: Okay. I have to say that I change the metaphor about once a week. But it may help if I give you an idea of how I go about writing.
I'm about 10,000 words into my next book. Do I know what it is about? Yes, I do know what it is about, it's just that I'm not telling myself. I can see bits of the story and I know the story is there. This is what I call draft zero. This is private. No one ever, ever gets to see draft zero. This is the draft that you write to tell yourself what the story is. Someone asked me recently how to guard against writing on auto-pilot. I responded that writing on auto-pilot is very, very important! I sit there and I bash the stuff out. I don't edit -- I let it flow. The important thing is that the next day I sit down and edit like crazy. But for the first month or so of writing a book I try to get the creative side of the mind to get it down there on the page. Later on I get the analytical side to come along and chop the work into decent lengths, edit it and knock it into the right kind of shape. Everyone finds their own way of doing things. I certainly don't sit down and plan a book out before I write it. There's a phrase I use called "The Valley Full of Clouds." Writing a novel is as if you are going off on a journey across a valley. The valley is full of mist, but you can see the top of a tree here and the top of another tree over there. And with any luck you can see the other side of the valley. But you cannot see down into the mist. Nevertheless, you head for the first tree. At this stage in the book, I know a little about how I want to start. I know some of the things that I want to do on the way. I think I know how I want it to end. This is enough. The thing now is to get as much down as possible. If necessary, I will write the ending fairly early on in the process. Now that ending may not turn out to be the real ending by the time that I have finished. But I will write down now what I think the conclusion of the book is going to be. It's all a technique, not to get over writer's block, but to get 15,000 or 20,000 words of text under my belt. When you've got that text down, then you can work on it. Then you start giving yourself ideas.
From an interview with Terry Pratchett from April 2000. http://www.writerswrite.com/journal/apr00/a-conversation-with-terry-pratchett-4001