I am surprised nobody here uses the Taryn spin method for filling cartos...
I hate the mess involved with the condom method.
I fill a Smoktech XL with around 50 drops (remember different droppers create different size drops).
I leave the condom on the battery side and add about 15 drops, then give it a few spins.
Repeat until you see a drop that has snuck into the condom.
Then I pull the condom from the battery end, wipe it off and blow from that end toward the mouthpiece end. redepositing that e-juice in the filler on top.
I usually let it sit in a shot glass for a bit, but can vape it right away.
After a bit of practice I barely waste more than a drop or 2.