Printer Cartridges are surprisingly costly. As an alternative, you can get it refilled at a store. The cheapest and effective alternative though is to refill it yourself. All it takes is a bottle of printer-ink and a syringe.

Step 1: What You Need

1. Printer-ink. You can buy it from a computer store. In Bangalore, you can buy it from an ink-refill shop at the entrance of SP Road, near City market.
2. A 1ml disposable syringe. The smaller the needle, the better.
3. Your printer cartridge. My printer uses a HP56 cartridge. But the refill method is same for most inkjet cartridges.

Step 2: The Cartridge

There is a black spot on the white-sticker covering the top-cover of the cartridge. Underneath the black spot is a hole through which ink can be injected into the cartridge. You can peel off the sticker to locate the hole. Or simply pierce the black spot with a needle ( as shown in the figure ).

Step 3: Refill Your Printer Cartridge

Draw ink into the syringe slowly and inject it into the hole. That's it!
Some important tips:
1. The needle size is very small. If you draw ink into the syringe too quickly, you can create air-gaps. Air gaps can block your ink cartridge. So make sure that there are no air bubbles, before you inject ink through the hole.
2. Refilling is a messy job. Make sure you have a piece of paper/waste cloth. The image has a clean background only to make it look good!
3. I was advised to fill the cartridge to near-capacity and not full. i.e., my cartridge's capacity is 19ml. So I filled it to about 13ml. Probably this is to prevent leakage.
4. Once, the cartridge is filled, it should be allowed to set for a few hours. This lets the ink to settle at the bottom of the sponge. The sponge is located inside the cartridge & cannot be seen.

P.S. Sorry for the poor image quality in this instructable. I ruined it by putting the camera too close to the objects.

Acknowledgement: I was "taught" to fill ink by my neighbor Sridhar. Thanks very much!