OhxMuffins pretty much covered it. All it is is a 2 liter coke bottle with a bag (lol, one from the wal-mart produce section that you put like tomatoes in) and some electrical tape. It was originally a gravity bong, but I added a bag because the only thing we had that held enough water was the bathtub (conveniently placed next to the toilet which you occasionally need to throw up in if you hit it right )

But yeah, cut the bottom off, put the bag inside the bottle (like line the inside completely, you don't want any air space between the bag and the inside of hte bottle. If you leave some you won't be able to clear it. Then just tape the bag round the part you cut off. If you make one post a pic