I just hold it with the juice just barely sitting at the top of the airway. When the condom is empty I fill it up again. My cartos are 1.8 ml so I had previously measured how much juice, in the carto, I would need. You could also buy a syringe and try filling down the sides of the filler. After cartos I moved up to a tank+ carto and am now using a clearomizer- so much easier IMO.
I probably tried to watch with too much going on around here - when it first appeared over the top I thought he said something about not doing that but then he keeps doing it so I thought he was pushing til it came to the top and then waiting and then doing it again. Well, that's good because mine never came out the top!, but quite a bit did still drip out the bottom. Just filled the rest with drips until it wasn't going in any more.