The clear plastic things on each end are just covers so stuff doesn't get in there during packaging...so, they are basically trash.
You can actually use the one that is flat on the inside, (the other has a nipple in it) fill that with some liquid and push the mouth end down into the clear rubber to put the liquid in. The pressure of you pushing the carto down into the rubber part will suck the juice into the cartridge.
Here check this video out, it's called the condom filling method heh