Hi Dean,
I used to use Greensmoke. The youtube vids for refilling Greensmoke are usually old. Greensmoke changed their cartomizers and now glue the end cap in. Its very hard to remove, and takes a fair bit of force - if you even can.
Vapor Kings sell cartos compatible with Greensmoke - see
here. These are refillable blank cartos and are cheaper the Greensmoke ones.
If you manage to get the cap off and drip the Greensmoke ones you'll be lucky. I tried on my old ones as an experiment and couldn't do it. You can condom fill them from the battery end but its a risk with the unsealed Greensmoke
batteries. The syringe method would be safer, but means mucking about with a needle.
Getting some compatible cartos is probably the best option. Greensmoke cartos are E9 compatible, and as well as Vapor Kings I think Health Cabin in China also sells them.