To refill ink cartridges for HP printers, you need to peel back the sticker and inject ink into the center of the cartridge. The amount of ink depends on whether you have a standard or a high yield cartridge. You can use a syringe to insert the ink into the sponge inside the cartridge. If you have HP Instant Ink cartridges, you need to replace them with new ones as indicated by the printer control panel.

Here are some general steps that you can follow:
1. Remove the empty cartridge from your printer.
2. Cover your workspace with paper towels.
3. Peel off the label on top of the cartridge.
4. Locate the fill holes on top of the cartridge.
5. Fill the syringe with ink.
6. Insert the needle into the fill hole and inject ink slowly.
7. Replace the label on top of the cartridge.