Step 1: Blagging cardboard from skips makes you look like a tramp but it's worth it.
I started out by liberating a huge slab of solid cardboard from the local tip, and blagging another one from a neighbour, then blagging a further 6 Mccoys crisp boxes. (make sure that IF you use cardboard with logos on, you keep the outward facing sides logo-free, so no logos can show through the material you cover it with).
The Pac-sides were shaped first, string and pen to get a good circle, measure a third in for the mouth, and... cut. You can see the intricately constructed shoulder harness in the above picture, which was made by hacking a hole in the top of a crisp (chips for you yanks..) box for my head to fit through and taping the whole thing to the side. Technical stuff eh? I can tell you're impressed.