The phrase “Write you what you know” has the obvious (and questionable) meaning of “write about what you know about, from your real life … (rather than making anything up).”
But I think a truer sense of the phrase is that you have to start writing before you know the whole story — once you have a few ideas, or inklings, or a sense of tone, atmosphere, etc, even a single compelling image, start writing. Write what you know, without expecting to know everything from the outset. If you wait to know everything about the story before you start writing, you’ll never start, because you can’t learn everything that way. You have to prove to the story that you’re willing to take the risk of committing your time and effort to it without the certainty that it’ll turn out well, or turn out at all.
The original idea for a story is just a way of getting into it — like a string you pull on the blank page that allows you to enter its interior. Once in this interior, things happen that you couldn’t have predicted, and you have to be willing to go with them if you want the story to take on a life of its own. And if it doesn’t take on a life of its own, the story is just thought, not art.
In the same way, when making an independent film, a lot of the time you have to start shooting without knowing you have the money to finish — the only way to get the money to finish is to have already started, to have already proven to yourself and others that you’re really going to do it, and to have something tangible that shows what it is.
Herzog says he’s done this on nearly all his films.