If you are burning your filling in the carto, best to toss them and get a new one. Burnt filler can linger if even with a refill of juice. Scorched, you can get around, burnt is just a toss. The second you get a burning flavor, it's time to top off. Don't even wait a couple of puffs, just top off. This way they last tons longer, and you aren't burning up your investment.
Refilling your carts is super simple easy to do! Just pop off the top and drop by drop fill it till it looks squishy inside. Not a pond, not a lake, squishy. You replace the top and off you go again

I have a big paper clip I use normally, but if I don't have that, I can just pop off the top with my fingers, and use anything straight and long to pull out the rubber o-ring looking thingy.
Are 20-23 drops for an empty carto, and 5-10 (depending on how empty it is) on refills.
Cleaning is just as easy. Pop off the tops and pull out the o-rings. Boil them 2-3 times in 5-10 min increments, rinse out real well, then blow them out and let them dry over the next day or so. pop the tops back on and you are done
Maintenance your battery as well by cleaning off the threads, and the interior ring.