Pratchett never sat down and wrote a how-to book, and he was wary of writers who did. What he left instead is a scatter of interview lines and offhand advice, a few of which have outlived him and get quoted at every writing workshop going.

The Direct Answer
Write a loose first draft to find the story, then make the book in the rewrite.
He treated the first draft as a private telling and the second draft as the real work.

The First Draft Is for You

The Pratchett line writers repeat most often is that the first draft is just you telling yourself the story. Nobody else is in the room yet. You're not performing for a reader, an editor, or a review, you're finding out what actually happens, which jokes land, which character refuses to stay minor. So you get it down, all of it, and you don't stop to buff a sentence you might delete tomorrow. Momentum first, polish later.

That permission, that the first draft is allowed to be bad, is the part people miss. A draft nobody will ever see can afford to be clumsy, overlong, and wrong in a dozen places. Its only job is to exist. You can't revise a blank page, and you can't tell yourself the story if you keep breaking off to fix the grammar.

He said much the same thing in blunter, funnier language when he described the gears he wrote in.

First draft: let it run. Turn all the knobs up to 11. Second draft: hell. Cut it down and cut it into shape.

Terry Pratchett

He usually described a third pass too, a lighter one he called combing its nose and blowing its hair, the final tidy once the shape was right. The exact number of drafts matters less than the order. Generate, then shape, then polish, always in that direction.

The Book Gets Made in the Rewrite

Look at where he puts the pain. The first pass is the loud, generous, everything-switched-on pass, and by his own account most of the book tends to hand itself over during it. The second pass is the hard one, where you sit with the sprawl and carve it into something a stranger would want to read. That's the step a lot of beginners skip or dread. They try to make draft one flawless, line by line, and stall out before the end. Pratchett ran it the other way. Finish the telling, then earn the book in the cutting.

There's a reason the ordering works. The generative brain and the critical brain pull against each other, and trying to run both at once tends to lock you up. Pratchett split them by draft. First he let the storyteller off the leash with the volume up. Then he brought in the editor to be ruthless. Each pass got exactly one job, so neither had to fight the other for control of the same afternoon.

He was just as quick to wave off the excuse writers reach for most. Asked about the block that's supposed to stop you, he refused to treat it as a real condition, and put it in one of his most-quoted cracks: there's no such thing as writer's block, and it was invented by people in California who couldn't write. It's a joke with steel under it.

If you've promised yourself a daily word count, block stops being a mood you wait out and turns into a problem you write through. The advice and the routine were the same tool, aimed at the same enemy, which is the day you never sit down. None of this was theory for him, either. He wrote more than 70 books on this method, so it carries a track record longer than most whole careers. Skip the hunt for the perfect sentence, get the story out, then treat the rewrite as the place the book is really assembled. For how that discipline ran in practice, day after day, see the full Terry Pratchett writing routine.

What You Can Steal