I know nothing about Green Puffer. What model is it?
Warning - if you have automatic batteries (no button) then you can kill your batteries with a
juice drip out the bottom of the carto. When refilling your cartos you need to be extra careful that your carto is not going to drip.
And note - if you ran your cartos dry to the point where you got a singed taste, your refills may taste singed
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Take a used carto and see if you can pop out the mouth-end cap by sticking an unbent paper in just far enough to get some leverage (not so far that you would catch stuffing or wires).
If the cap is soft and comes out, look for a possible inner donut-shape silicone seal that may need to be removed.
Always take the carto off the ecig before adding
juice
I hold a tissue piece on the threaded end hole, tilt the carto and drip
juice in the the stuffing
If you want to stick a syringe down between the inner wall and the stuffing you could do that instead and it might be safer with your auto batteries. But you still need to do cleanup.
I clean up any drips at the threaded end totally
Tilt the open end down into a tissue and puff into the threaded end to clear the air channel (if the carto has a defined air channel)
Use the folded tip of a tissue to clean up any loose juice pooled above the stuffing
Cap
If the carto was very dry or blank when you filled it, let the carto sit various sides to make sure every bit of stuffing absorbs juice
If the cap is hard ("name" brands like to used harder to remove caps) hopefully you can get it out
If the cap is glued in (another "name" brand trick) it will be hard to get out
If you have carto condoms and hard or glued-in cap, you may need to use the carto condom to push jucie into the carto. I really don't recommend that with auto batteries because you can do a complete cleanup. If you need to use this method, IMO buy a spare battery. But first find out what model it really is because Green Puffer is a HUGE ripoff on parts - they are charging $50 for a battery that should cost $15. They are probably either a KR808D-1 or the less supported KR808D-2.