I used xl e2 cartos, and tried the "condom" method, with poor success, so I found another online tutorial on youtube and it showed exactly how to take off the soft tip and the little silicone ring, then find the two small holes on either side of that little ring in the carto, the filling kit I got has little white needle looking plastic things, that if you are very careful won't bend in half, and I was able to fill the cartos in that manner. you just have to remember to fill slowly and to fill the syringe slowly too as the liquid is very thick.
someone on chat also suggested I shake down the carto after filling, like a thermometer(if you aren't a mom, this is kind of hard to explain...) and that seemed to help.Also seems to help if you turn the carto upside down when you are done and blow hard through the little hold, then dab the mouthpiece with a napkin or tissue to get the residue off, since I started doing it that way, I have had much happiness with my cartos