My familiarity is with KR 808 cartomizers (IKV LR Mega 808s are my favorites) and not 510s. IMO it's too impractical to fill them from the bottom since they are male end. Therefore, since they hold about 1.5 mls, carefully remove the soft rubber cap with a toothpick/bent paperclip, fill the condom with 25 drops of your favorite e liquid and slowly press the open end of the cartomizer into the condom. Let the liquid run down the cartomizer/soak the filler. BTW, almost forgot, keep the male end sealed. Let the carto settle for atleast a half hour or more. Hope this helps... I recommend keeping 1 flavor per carto. In the grand scheme of things, they are cheap!! Have fun and vape your head off...