Ambient Sound Design Tips?

I've only sound designed one other show and it was a musical where most of the stuff needed were just clear sound effects written in the script but for this show there is a lot more freedom for sound design and I've decided I want to create a soundscape of a old run down theatre for the show, but I don't know much about the basics of that aspect of design, are there any do-s and don't-s I should know?