The condom method is when you fill the little rubber sleeve the carto comes in with juice about half way. Slowly push the carto in battery side down into the juice. It forces the juice up into the carto. Then add some drops to the top.
I usually add about 5-10 drops, shake like a thermometer, add 5 more, shake repeat until it's fully absorbed. Get yourself a drip tip so it's easy to top off your carto with juice. Once you get that burnt taste the carto is usually done. You have to keep the carto wet.