Most printers will notify you with an alert on the display when your ink is low and when it's time to replace your cartridge. For older models that don't have this function, broken text and white streaks on prints is an indicator that it's time to swap for a new cartridge.
When that time comes, first ensure the replacement cartridge you're using is the correct model for your printer. You'll want to follow your specific printer model's instructions on how to put ink in a printer. These instructions can be found in your printer's user manual or on the manufacturer's website.
First, ensure your printer's power is off before ink cartridge installation. For many printers, you will first open a top or front door on your printer. The carriage that holds your cartridge will, for many models, center itself automatically for easy access. Remove the empty cartridges, which are usually held in place with a latch, and replace them with a new one, which should click in place to indicate it has been properly installed. Close the printer door and power on the printer.
For your empty ink cartridges, we recommend recycling them at a local office supply store so they can be used in the remanufacturing process to reduce waste.