This Page Will Give You A Brief Overview Of Refilling The Canon 4000 Series BCI-21 Color Cartridge

First off you will need to remove the cap from the top of the cartridge. This can be accomplished using pliers or a vise. It will snap off under light pressure.
A modification must be made in order to gain access for refilling. Use the drill tool provided to open the existing vent holes. Opening the vent hole diameter will not affect cartridge performance.

After drilling all 3 vent holes you may now refill the cartridge using the provided syringes and blunt tipped needles.
Tape the cap back on after filling all the colors. Don't cover the small vent on the top of the cap! It must be clear in order for the cartridge to breathe. Complete instructions come with the kits.

Thats it! Always check to make sure that the cartridge does not leak before installing!