When dealing with these guys:
http://www.madvapes.com/media/catal...f78eab33525d08d6e5fb8d27136e95/3/3/3368-1.jpg
Best to refill them while on your PV so they are held down.
While sitting at a desk or table:
Remove the drip tip.
Twist the tank back and forth (clockwise and counter-clockwise) while gently pulling up, until the top of the carto is about 1/8" below the seal.
Using my lfet hand. I push on the left side of the tank with my index finger so that it touches the carto. This opens a gap on the right side.
I tilt the PV to the right and place juice bottle standard tip between gap and pointing towards the tank and away from the carto.
Fill to just below top of carto, set juice bottle aside.
Place PV/tank/carto in front of you so you can "look down the barrel".
Make sure you hold the PV in one hand and the top of the cartotank and seal with the other (failure to do so can cause a freshly filled tank to dump it's contents.
Line up the carto with the seal, using the same back and forth twisting motion, push tank down onto carto.
Make sure that the flange at the base of the carto inserts into the tank bottom.
Replace drip tip, cap juice bottle, go vape.
Literally, I can do this whole process in 10 seconds including uncapping and recapping my juice.