Cartos?
In general for any carto -
First, you don't vape them to the point where they taste singed because singed flavor will not go away.
You gently try to pop out the mouth-end cap by sticking an unbent paperclip in just far enough to pop the cap and not so far that you catch wires or stuffing. If the cap is hard to budge it may be a hard cap or worse a glued in cap (rebrander trick to force discourage refilling).
If you do get the cap off you should either see stuffing or stuffing with a donut shape silicone seal over it - if there is a donut, carefully take that out. Hold a tissue on the threaded end and drip juice into the stuffing. Clean up the threaded end. If there is a defined center air channel I would blow into the threaded end with the open end pointed down into a tissue. If there is juice loose above the stuffing, sop that up. Make absolutely sure it is not going to drip because if you have automatic batteries you can easily damage the drag sensor or kill the battery if a carto leaks. Recap and use.
If the cap will not come off, then you are forced to use methods that I think are more dangerous for use on auto batteries since you won't be able to see inside the carto. You can push juice in with a carto condom (assuming your cartos even come with those on the ends). You can try dripping into the battery hole (obviously dangerous if you plan to use the carto on auto batteries).