Different 510 cartos can use different methods. If you use Joye 510 cartos they recommend using a needle but that is not neccesary. I drip drops into the battery end hole one drop at a time, but it doesn't take long at all to fill or refill. I was amazed at how easy and mess free it was to do it this way. I've not tried the BOGE 510's with the soft caps yet but have read that you can remove the outer and inner caps and drip onto the filler from the top (mouth piece end) pretty easily. Lots of different methods out there so experimenting with which type of 510 carto and method of filling works best for you and hubby will let you find out which you prefer.