I just started vaping, but I've been doing my homework =) Here's some good videos to watch for info on refilling cartos.
(carto cap filler)
(filling from either end)
I don't know if this works for a 510 carto the same as the 808, but there are videos on filling 510 cartos too.
There's also a video on dripping a carto - haven't tried it yet, but it couldn't hurt to check it out.
Good luck!