I use the Joye 510 cartomizers. I refill them with two methods, neither of which uses a syringe or requires disassembly of the carto. I will be trying the syringe method in the next week or so.
To refill by condom method, first the cautions... the "condom" caps on these cartos is not nearly as snug as I'd like, or as snugly as I've seen on some other brands. And they are not very tall. Nor are they particularly sturdy, so you have to use a little care when inserting the carto to prevent buckling. So start slowly when using the condom method to fill or refill these cartos.
Start by filling the taller cap half-way full of juice, maybe a tad more than half. Make sure you aren't blocking airflow through the carto at the mouthpiece end. Slowly insert the threaded metal end of the carto into the condom. As you force the carto into the condom, it forces the juice into the tiny holes in that tip. I say holes, plural, because there are breathing holes in the sides of the threaded tip, and one in the end, for a total of three. As your carto hits bottom in the condom, you can squeeze the tip at the bottom of the condom to force more juice into the carto, getting a few more drops in there. Turn it upside down and stand on the mouthpiece end if possible for a few minutes to let the juices seep in. Or better yet, just pull the carto back out and wipe it dry with tissue, then put clean dry caps/condoms back on both ends and let it soak up awhile, maybe overnight. Then use this same condom to refill a batch of dry cartos. That way, there is little waste, since those last few drops that don't get soaked up will be passed to the next carto. At the end of the batch filling, those last few drops can be poured back into the bottle or dripped into something else.
If you used too much juice and overfill it, you'll see juice pooling out the mouthpiece end. You can then blow that excess out of the mouthpiece onto a paper towel by blowing into the threaded end forcefully like blowing a whistle. You'll get a little blowback juice in your mouth this way, but not nearly as much juice in the mouth as when you just start vaping a saturated carto. You can also try the "thermometer shake" onto paper towels to get out the worst of it. I just bought a plastic syringe without a needle, the kind used for giving infants liquid medicine. It came with a little rubber cone that fits over the syringe tip securely, and the other end is a perfect fit for the 510-size cartos. You can then use the syringe plunger to pump out the excess, which means getting no juice in the mouth!!! <--- three exclamations to show my huge enthusiasm for NOT getting juice in the mouth!!
The drip method for these Joye 510 cartos is easy. Just hold the carto with the metal end upwards. Use a bottle that dispenses juice one drop at a time if possible. Place one drop on the hole in the metal tip. It will soak into the carto in about a second. Place the next drop, it soaks in, the next drop... about as fast as you can place the new drop, the previous drop soaks into the metal tip. You can either count drops, some say about 15, or you can keep dripping in until you see excess coming out the mouth end. You'll want to avoid that overfilling if possible.
And one more note. I have been buying my Joye 510 cartos from Cignot.com and have received outstanding customer service. But more importantly to me is that I have bought the cartos pre-filled, and the juice inside was great! When new, the cartos seem to go a long while. Not quite as long on the refills, but that's probably due as much to refilling methods as much as anything.