Sorry I've been away for a bit. Here's my method of filling... what works for me may or may not for others, but it never fails for me. Using 100% PG based juices only, works much better anything with VG in it. Trust me, I've tried a lot of blends, 70/30 and 80/20, and went right back to my PG ONLY juices. I know other people say they use blends... but it simply does not wick fast enough for me, not to mention it really changed the flavor and throat hit of my juice... and not in a good way either! OK, now that my PG/VG rant is over I can explain my method.
First, I remove both of the 'condoms' and the little 'buttplug', then put the smaller of them back on the threaded end. I typically push it almost all the way on... but leave a small space. Taking a juice filled syringe, with a fairly small needle, I poke it in to the filler down to the bottom of the carto. When adding juice, gently push on syringe while slowly drawing it up. Repeat this process a few times until filled... don't worry about any juice that comes up through the center or overfilling either. After it's filled to top I take it and give it a good 'thermometer shake'... so that there is a small amount of juice seen in the carto. You should notice that the filler material looks a bit dry after doing this. This helps to spread the juice all through the filler material much better. I then add enough drops till it's to the top of the filler material again and repeat process. Now, the most important part from what I've learned, is to stand the carto up in a shot glass or something (carto still on) allowing it to soak in totally. I prefer to leave it overnight since I already have cartos I'm vaping on. After it has sat, for however long you can let it, it's ready to vape. Remove condom, wipe off any excess juice from it, throw on a drip tip and vape! And as other have said... I toke on it for 10-15 times and set it down for a bit. When it's time for more, I just pull drip tip off and add enough drops till it's nice and wet, and vape away. Some would say: "Why the hell don't you just use an atty then since you have to drip often?" To that, my response is that I actually prefer the flavor/vapor/throat hit of my carto over an atty anyway. And the other reason is that when I did direct drip, I was constantly having to wipe juice up that leaks from the bottom of atomizers. With my cartos... there isn't NEARLY as much of that, rarely actually. It's a hell of a lot easier to over drip and get those nasty 'dry hits' if it's not properly filled. And don't believe em when they say: "I never get 'dry hits' or have leaky atty's"... there lies I tell ya, all lies! Attys just aren't for me... and with cartos you don't have to worry about harsh hits. Especially, if you take the time to 'top off' often, it provides the best tasting/most constancy while vaping for me anyway. Experiences and opinions obviously vary greatly in the 'carto vs. atty' debate... just try to find what works for you and you will be totally satisfied!
Hopefully I didn't leave too much out... I don't know how I could have with the length of this book!