Prepare a cartomizer
Remove the silicon covers from a fresh cartomizer and set one of them aside for later.
You should probably also have a paper towel handy, as it may get a little messy. Once you've had some practice you won't need one.
Pop the top
Remove the mouthpiece from the end of the cartomizer. You may need the assistance of a small tool like a paper clip.
Insert the tool through the airhole and pull it at an angle to pry the mouthpiece off.
Fill it up
Fill the silicon cap with your favorite ELiquid. Push the cartomizer into the cap with the open mouthpiece end facing down.
Flip the cartomizer rightside up with the endcap still attached before slowly removing the endcap.
Let it settle
Let the cartomizer sit for 5 - 10 minutes to soak up all the liquid.
You may need to blow air through the mouthpiece end to blow out any excess liquid. Wipe off the battery connector with a paper towel and put the mouthpiece back on the cartomizer.