You mean carts that push onto an atomizer? If so, the cotton/stuffing is required. The stuffing holds the
juice, the atomizer bridge /\ has a wick, the atomizer bridge ends up pushed into the stuffing and the wick wicks juice from the stuffing to the heating coil under the atomizer. You add juice to the stuffing. On a full fill you can remove the stuffing, fill 1/2 way, pushing the stuffing in and down into the juice, reposition the stuffing at the top edge of the juice cup and top off with 1-3 drops so it just looks damp on top and not a puddle. If it is a recessed juice cup then you need to get it back down into the mouthpiece without pressing on the stuffing. Then whle
vaping, if the atomizer is getting hot or the vapor is light or tastes bad, then check the cart, add a few drops if needed to make it look damp on top, make sure the stuffing is near the top edge of the juice cup. Carts are a bit of a pain. Many people veentually switch to cartomizers which hold more and need juice less often but have limited lives.
After a few days the stuffing may get gunky and I recommend cutting blue foam stuffing for it instead. (I think ModernVapor sells some small foam pieces, strictlyejuice.com sells piece precut for 510, Petsmart sells the uncut as Marineland Rite-Size U Bonded Foam Sleeve for Magnum H.O.T. filters.)
OR if you mean you actually have cartomizers aka cartos which screw onto the battery (a 2-piece ecig) then they are filled differently (you cannot remove the stuffing, you cannot restuff them). What do you have???
Maybe you should watch the Newbie videos in the sticky threads of New Members. I think they show how to fill carts and cartos?