All you need to do is to become the user postgres (assuming that
is still his name in Debian) before running those commands.
Not that hard, really.
That said: the document you followed is not very
detailed, really; it fails to mention which user you're
supposed to be running all this as, and where to
create the directory structure.
On a second note: the document uses an ancient version
of postgres. If danbooro hasn't changed since then I'd
recommend not using the product at all - the current
branch is 8.4.4 (8.1 is still maintained, but the changes
in performance at quite substantial).