as far as i know, you just leave a microphone and recorder in a quiet, empty room and let it record. i guess if you record for, say, 2 hours, you have to go back and listen to the entire two hours of silence and hope you hear something!
i remember seeing a television show about it several years ago, the show itself was from the late 70's or early 80's.... and the people doing the recordings were just using those older portable tape recorders, so you don't really need any fancy equipment.