When Vonnegut gathered his old magazine stories into Bagombo Snuff Box in 1999, he opened the book with eight rules for writing fiction. They've been quoted, taped to desks, and argued over ever since.

The Direct Answer
Eight rules, and the first one outranks the rest.
He published them in the preface to Bagombo Snuff Box (1999), then admitted the best writers break almost all of them.

The Eight Rules

Vonnegut framed the list as advice for writing a short story, though it applies to almost anything. Here they are, in his order:

The Rule He Cared About Most

Right after the list, Vonnegut undercut himself. He said the greatest American short story writer of his generation was Flannery O'Connor, and that she broke practically every one of his rules but the first. That first rule, about respecting a stranger's time, was the one he refused to let go of.

The greatest American short story writer of my generation was Flannery O'Connor. She broke practically every one of my rules but the first.

Kurt Vonnegut, preface to Bagombo Snuff Box

He liked rules the way a carpenter likes a level. Useful, not sacred. For how he actually built his sentences, see the full Kurt Vonnegut writing routine.

What You Can Steal