Cartos have to be saturated to vape properly. The coil is at the bottom threaded end of the carto. If you only fill from the top, it takes time for the
juice to soak to the bottom and you will get burned taste. That can ruin a carto.
Use a blunt needle syringe that you can get at the Pharmacy to fill the carto. Fill the syringe with juice and insert the needle between the barrel and the polyfill filler. Push it to the bottom and draw it back as you fill slowly. Repeat on all sides until the top looks slushy.
You can also use the condom fill method with the long end cap that comes on the carto. Fill the cap about 3/4 full of juice and slowly insert the carto, threaded end down, into the cap. Let it sit for 30 minutes or so until all the juice is absorbed.