Ideally, it should be pretty easy though I've yet to make it work, though I haven't played with it much, as I quickly upgraded to the eGo.
Inside each orange cartridge is a small, clear plastic 'holder' with some polyfil in it. Take the piece of foil out and toss. You should be able to just fill the clear holder with
juice and stick it back on.
You might want to sacrifice one cartridge to experiment with, ie. try it with or without the polyfil in it, try it without the clear holder, etc.
sorry, that's the best I can do