I do exactly what pmos said: remove the cap and drip around the edges. I usually do about 8 drops, and let it sit on an angle (or upright, but it's more likely to leak out the bottom battery hole that way) for a few minutes. You'll see that the filler is pretty much dry again, so another 8 drops or so, let it sit again (3-5 minutes) and top off with about 4 drops. That's about as much as I usually fill them, or it ends up running out the bottom. Quick blow through and dab the battery end and they're ready to vape.
I don't really want to list all the juices I've tried, but anything 70/30 PG/VG or thinner works fine. Gardner hit on some good vendors. I really only order tobacco flavors. Some heavier PG/VG blends are thinned to make them vapable in BMF cartos.
Backwoods Brew is good stuff, but it has to steep at least two weeks, in my experience. Out of the mailbox, it's like perfume for the most part. After steeping, good, rich flavors, although everything I have is tobacco-based flavors.
Halo stuff is thin as water, and vapes very well in BMF cartos.
Johnson Creek PG blend is very thin too, but I can't stand the stuff I've tried. Very expensive 'yuck' juice, IMO.
VAPOR STATION! They have great stuff, it's not very expensive, and they use glass bottles with glass drippers. Tampa is very analog-like. Very clean flavors from them.
And after all the juices I've tried, I'd trade it all for a big bottle of Bloog's 555, or even RY4. I can always transfer it to an empty glass dripper bottle.