Do you really mean carts (cartridges) - i.e. the plastic and stuffing piece of a 3-piece ecig? Or do you mean cartos (cartomizers) the mouth end of a 2-piece ecig? (regardless of what your vendor called them)
If you have carts, there are two basic kinds...
- The kind that sticks INTO a recessed atty - on those carts you can see the stuffing and just need juice in a dropper bottle
- The kind that IS recessed and the atty sticks INTO it - with most of these you need to stick an unbent smooth small paper clip into the mouth hole to push up the inner juice/stuffing cup for filling. Then you have to press the juice cup back in without pushing its stuffing down deeper into the juice cup (the stuffing should be near the top of the juice cup to work). My little carts come with a soft plug and I carry the plug with me and use it to push the juice cup back into the mouthpiece. If you don't have those, you could try to push the edge of the cup with the paperclip or cut a piece of dowel that just fits in the mouthpiece and use that. If you push the juice cup back in with the atty you might push the stuffing in too far.
Normal cart filling method - complete fill is remove stuffing carefully, fill cup 1/2 way, replace stuffing pushing it into the juice a bit, reposition stuffing, and top off with 1-3 drops until top looks damp but not a puddle. if your cart stuffing looks like separate vertical fibers, that may be hard to pull out and replace, so just fill slowly from top. When my white stuffing starts tasting bad or wicking badly I replace with blue foam if possible (some cart shapes don't work well with blue foam)
If what you have are cartos, what model? You could probably find a video on Youtube of how to refill them. But if you bought a rebrand from a fancy website, sometimes they use hard caps or worse, glued hard caps, to make it harder for you to refill them.