I have used both types of cartos, the regular ones with filling, I just pop the top off and drip till the filling looks a little mushy, but not swimming. flick it down a few times, like you are shaking a thermometer, and top off. Pop on the cap and vape away.
The ceramic ones (CE2(v4), I pop off the top, remove the inner ring(promptly throw it away) and use my syringe to fill it, or, my paper clip to pop that last ring/cap out and drip fill it. Replace cap and top and vape myself silly.
The reason I moved over to the ceramic ones is because the ones with filling, if they get a little dry, they burn. After pulling one apart one day and seeing a huge burn hole in the center of it, was the last time I used a carto with filling in it.