There are so many filling techniques, but the one you choose that completely saturates the cart filler is best.
The folks that use a syringe with needle fill from the bottom up, mucxh like is done in the manufacturing process. It gurantees you get the most liquid in your cart.
Some drip some liquid in the cart, stuff the stuffing into the cart, drip a fe wmore drops, pull the filler up to the top, restuff it down to the bottom, drip some more, and repeat as necessary. There are smushers. This technique takes mre time and manipulation, but eventually produces a cart fill nearly as full as a syringe user.
Lots of folks just top off with three to ten drops (depending upon the device you are using). Sometimes the toppers will go with the same cart for weeks at a time. They never really refill a cart but keep it wet enough to procude acceptable vapor.
So you an use a syringe to fill from bottom up, smush or top off indefinitely. Certainly more advanced vapers will chime in with refilling methods I missed.