hey zombie,
one additional hole on the opposite side should do. that'll give you two holes. the suction created by you
vaping it will force-feed the
juice into the carto to some extent. if you're using some ridiculously thick juice (think motor oil), then two additional holes might be necessary but i'd start out with just the one additional. if you're using a really thin juice (with a consistency close to water), you may want to try it with just the one little hole that's provided. if you don't get a burnt taste (meaning the carto's running dry), then you'll be fine like that. i just leave the one single hole on most of my PG heavy juices and it works just fine.
and the white rubber cap can be trashed and replaced with a drip tip. if you plan to use the carto without the tank, then the rubber piece *could* stay in place instead of the drip tip, but where's the fun in that?