@franfritz on most Canon printer you start by opening the lid on your printer. Let the ink or toner cartridge slide completely out of the way. You will now have free access to the waste ink absorber.
Detach the waste ink absorber by pulling gently on its rubber frame. Pull it away from the printer to remove it completely.
Unplug the printer to prevent mishaps.
Remove the foam pads and place them in a bucket of hot, soapy water. Let them soak long enough for the ink to get loose. You should be able to scrub it away easily. Allow them to dry completely.
Reattach the foam pads and install the waste ink absorber again. After this you will have to reset your ink absorber counter. Use this SM to work on your printer [document|3424]