You fill up a carto per the instructions for the condom method. Replace the plug in the top with a drip tip. As you vape it, every 20-30 hits or so, or every 15 minutes, you drip a few drops into the top through the drip tip. This way you won't have to pull that rubber plug out every time. While you are learning, it's best to take the drip tip off so you can see how the juice is absorbing. After a little practice, you'll know how much to add and you can just drip it through the top. You want to keep it wet. If you burn a carto, it's toast. Throw it out. When it's full, the top will look like a slushy. When it starts to get dry, the vapor will get thinner and weaker. That's the time to top it off. Don't let it get so dry it burns the polyfill inside. If you burn the polyfill, you'll know it cuz it tastes like .... Count on burning a couple while you're learning. Everybody does it.
When you top it off, tilt it so you drip juice down the side of the tube into the polyfill instead of dripping it in the hole in the center. When you get them and see them, it'll be a lot easier to understand. It's harder to read or write about it than to do it.