The condom or push method is the process where you take a silicone cartomizer cap with no center post on the inside of it and fill it with e-liquid. Basically a silicone bowl. Fill the cap with 10-15 drops of e-liquid. Slowly push the cartomizer, connection end first into the e-liquid filled cap until it reaches the bottom of the cap. Blow out any extra e-liquid into a paper towel and clean the end of the cartomizer where it connects to the battery.
The Drip method is probably more common and it requires dripping e-liquid into the cartomizer from the top. If you're using Blank or Prefilled cartomizers, then they will have a soft cap that can easily be pried off with a paperclip or straight pin. If it's a disposable (Prefilled) cartomizer, then usually it's a plastic cap that clicks on. With a little effort a paperclip or thumbtack can snap it off. If it's glued, you may have to use something stronger like a drill bit. Once you get the cap off, on some brands, you will find a silicone ring that you will need to remove. Tilt the cartomizer at an angle and begin dripping about 20-25 drops of e-liquid around the sides avoiding the center hole. When the polyfil material remains damp, your cartomizer is full.
If you accidentally get some e-liquid in the center hole, just blow the excess into a paper towel and clean the end connection with a papertowel or Q-tip. Replace the silicone ring if it's included and re-insert the top cap. It will snap back on. Wait a few minutes for the e-liquid to soak into the filler material before vaping. If you get a burnt taste or the polyfil material has brown spots, then the cartomizer is trash. Don't waste your time refilling these as they will taste nasty and will waste e-liquid.