You can thread it through a bit of tubing, so you have a figure-eight of bungee, with your neck going through the big loop and the DV mouthpiece going through the smaller one.
However, since my backup is for my use and mine alone, I knotted the ends of a bit of bungee, and put it under the cable tie that holds the mouthpiece on the DV. Very secure!
There's a couple of pix at
http://diveweb.oneandoneis2.com/necklace.htm that might make it clearer..