These cartridges are similar to other combined sponge/print head cartridges from HP and Lexmark and other manufacturers, and the refill procedure is the same. Here is an instruction from druckerchannel.de, and here is a Google translation into English. These Canon cartridges are older, but the design is the same as the PG-245/CL-246 cartridges, only the capacities might be different.
Druckerchannel mentions that the capacity of the black cartridge drops with more refills. It seems that they do not put the label back in place after refill. This could lead to the cartridge drying out, as the vent labyrinth, designed to minimize evaporation, is bypassed. If the label won't stick properly, it can be replaced by a piece of tape.