I've never done one, but from what I understand ...
you kind of open the foil rim
pull out the cardboard cover
use that cover as a pattern on your cs
decorate your cs
put your new decorated cover back on the jiffy pop
push the foil back down around your new cover
I would think, although I've never seen it written here, that you
could just decorate the actual jiffy pop cover.
Since I've never done this, though, I'm not positive.
You should be able to do a search of the forums to get more info.
Good Luck!