I guess -- it's just how you say it, you have to learn it.
I'm not sure it's called an "idiom" because I don't see what the (different) literaly meaning would be, but I think it doesn't matter, this is just how we say it.
Alice:"Wie schreibt man selbstständig?" Bob writes something. Bob: "Selbstständig wird so geschrieben."
Better: "So wurde es geschrieben" keeping the emphasis on the method - IMO