To refill an ink cartridge for an HP printer, locate a hole in the cartridge and use a syringe to add more ink. Exact instructions vary depending on cartridge type.
The hole in the ink cartridge is usually either on top of the cartridge or behind the label. Some ink cartridges require making a hole in a "dimple" in the cartridge and resealing the dimple after refilling the cartridge with ink. Other cartridges require accessing an ink bag inside the cartridge and using a syringe to fill the ink bag. Run cleaning cycles after inserting the newly refilled cartridge into the printer.