This is how I fill them.
I carefully pop out the white cap with an unbent paper clip, being careful not to catch anything under the cap. While you are filling a blank carto you need to hold something against the bottom hole or if the carto came with a tiny plug for the bottom you can put that in and the threaded and condom. I just hold a tissue against the threaded end hole. Shake your eliquid too.
Then you need to put in 18+ drops into the stuffing (approx min number of drops to equal 1ml since they are 1ml cartos) - tilt the carto and take your time. You can rotate the carto to drip juice on different areas of stuffing. Avoid dripping juice down the middle. Very very thin juice like 100% PG will try to leak out the most. I use 70% or 80% PG juices. When done making 18+ drops go in, clean it up. Dry the threaded end enough to put your mouth on it then tilt the open and down into the tissue and puff into the threaded end to clear the central air channel. You want to see light through there. Use a folded tip of tissue to clean up and losoe juice above the stuffing (if you don't, it will end up on your battery or in your mouth). When done cleaning cap it and lay it on different sides for 15-20 min to settle it and spread juice around so every bit of stuffing can absorb some juice. (longer if you are using thicker juice than I use). Then use it.
I do not use condom method, I don't spin to force more juice in. I just fill them as intended. I am very careful but occasionally I do get a small leak, especially with a more vertical and hold like I do with Riva. I try to hold the ecig more horizontal when vaping a carto that just had juice added. if you have auto batteries or particularly large drops you could try just using 16 drops to fill a blank but don't skimp on that settling period.