OK, plug your Canon MX420 into a USB port and turn on the printer.
Now in your web browser, go to this address:http://localhost:631
That is NOT the internet, it is the local CUPS interface for setting up your printer.
(if at any time you are asked for a username and password, it's your Linux username and password it wants)
Click the "Administration" tab.
Click the "Find New Printers" button.
When it finds the printer .. select these as the drivers:-
Model: Canon PIXMA MX420 - CUPS+Gutenprint v5.2.8-pre1 (en)