Just like it said, layer it about 1inch think around the entire box. (on all the sides)
If it is vented, don't block the vent...The polyfill helps absorb waves inside the box, allowing fit to have a better responce.
Also it seems like they're describing more of a matt type of material but poly should work its just getting it to stay in place. I couldn't tell you any techniques for doing so because I don't have all that much experience with it.