Canon ink cartridges, such as those for Canon's Pixma printers, can be refilled but usually require a chip reset device and some technical knowledge. Many people choose to have ink cartridges refilled by professionals because of the time and effort required, as well as the mess that refills can make.
After finding the round plastic stopper that the ink enters into, often underneath the cartridge's label, one must inject the fresh ink into the empty cartridge. This is usually done with a special syringe that is typically included with ink refill kits. After injecting the ink and making sure the cartridge has been sealed, insert the cartridge into the chip reset device and wait for a flash or beep. The cartridge should now be ready for use.