Drying boiled cartos for days may not dry them. Some have to bake them in a low oven (like 175F) for hours to get them dry. And note - if these cartos were heavily used before cleaning they will never work as good as new because you cannot clean off the baked on residue on the coil.
As for filling - with a DRY carto - you have to block the threaded end hole with finger+tissue or the carto spike from the carto condom. Then I make a new fresh 1ml carto take 18 drops of juices. Then I wait 20 seconds or so, then add however many more drops it will soak in (usually 4-8). (OR you could use the syringe - I don't). Then you clean it up - the threads, and then I hold a bit of tissue against the stuffing and tilt the open end down and puff into the threaded end (between light and medium single puff) to sop up any excess
juice. Then I settle the carto - I lay it on different sides and both ends over about 15-20 minutes (enough time for the 80pG/20VG thin juices I use to dampen every bit of the stuffing).