You put as much juice in as the filler material in the cartridge will hold without pooling at the top of the filler material. If it pools at the top, just dab it with a paper towel or tissue.
The problem is, if you are just putting drops right on top of the filler material that comes with the cartridge, it isn't going to sink to the bottom without a little help. You can use the straw modification, or you can use a toothpick or paper clip to push the filler aside a little bit so that it can breath while you are putting in the drops. Some people use a syringe or a needle tipped dropper bottle to get all the way down to the bottom of the cartridge.
And I have found that if you replace the stock filler material with blue foam, you can actually put the drops right in if you tilt the cartridge to the side while doing so, but that is probably a bit more than you want to bite off and chew right now.