Here is how I fill and I ONLY use thin clear juices. Thick juices will give problems. Juices that drop a ton of flavoring sediment in the stuffing will give problems over time. I don't use any tricks when filling. Once you do it one time it's easy. I just use a lot of detail below.
You can use other methods to fill but you still have to fill the carto to its stated capacity, clean it up, settle it, keep it damp.
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How I fill a cartomizer (Vapor4Life/Bloog/Volt for KR8 or Boge 510 for 510 which are all soft-cap w/ no inner seals and defined central air channel)
I use thin (20% or less VG) liquid or Vapor4Life WOW liquids, preferably clear liquids w/ no sediment
Filling a blank carto - I pop off the soft cap (unbent paperclip carefully in the mouth hole, not catching any wires or stuffing!). I hold some tissue against the threaded end hole (or you could use the carto condom spike to plug the hole). I tilt the carto and slowly drip 18 drops (per ml of carto capacity and these are 1ml cartos, and you may need to wait extra time for the first few drops to get the stuffing wet). I drip onto the inner wall of the carto so it slides down into the stuffing while I rotate the carto. Then I wait 15 seconds or so then I drip however many more drops the stuffing wants to soak up (usually 4-8).
Cleanup: Then I do clean-up. If it is leaking, I clean that up. Then I tilt the open end down into the tissue and puff into the threaded end to clear any juice out of the central air channel (I want to see light through there). With the carto still tilted with open end down I gently touch a bit of tissue to the stuffing (otherwise that liquid will be gurgling or spitting in my mouth). (And instead, if I really want to avoid leaks I touch the soft bit of tissue to the stuffing while puffing into the threaded end.)
Settle: Then since this was a fresh fill of a blank, I now let it settle. I lay the carto on different sides and the ends over 15-20 minutes. Then it should be ready to vape. If you use thicker juices than me, then you may need to let it settle longer.
As I am vaping - if the vapor gets a bit light or a bit dry or tastes bit off or the carto is getting unusually warm, I will take the carto off the battery, open it, add 4-8 drops or whatever it seems to want (I will either hold a bit of tissue against the threaded end hole just in case, or skimp on how many drops I add to make sure I avoid drips if I have no tissue). I check to make sure the threaded end is dry, give it a couple of puffs into the threaded end to make sure the air channel is clear, and continue to vape.
Note - it is easy for me to "open" my main cartos for adding juice because I replace the white end cap with a plastic or delrin drip tip (I get them at LiteCigUSA.net). So I can easily pull that out with my fingers when I add juice and it keeps my lips cool if the carto is hot. I can also pull off the drip tip to check if the top of the stuffing has gone white and could use some juice.