When I ran a small printing business, I regularly refilled customers' ink cartridges. You must get the right ink though. Although it is possible to buy general purpose refill inks they are usually pretty poor and van ruin the print heads.
If you get the right refill inks for your cartridge, they usually have instructions in the box about where the refill holes are located and how to do it.
Beware, the inks are messy. Wear disposable gloves. The supplier I used, provided them with the refill kit.
Not an advert but I used to use:
http://www.systeminsight.co.uk who also doa range of equivalent cartridges cheaper than the manufacturers' and just as good.