It's hung there with a couple of short sheet metal screws, you just need to avoid any wiring as I believe if you have an A/C unit the wiring is in the roof. We didn't have it so that wasn't an issue. It is light weight metal chain from the local home improvement store, and the top shelf is a closet rack turned upside down. The bottom 2 shelves were from Target - they stack together for easier traveling and just hang from the first one. The first one collapsed enough to just leave it hanging while traveling.
I've seen a bunch of different ways to do it -- here's one guy's way from arveeclub.com:
More shelves over the sink....my first mod and I am so proud.