how to install a canon mg3120 to an ipad mini

You don't install anything.

If the iPad is connected to the same wireless network the Printer is connected to and the printer is AirPrint compatible (which accoring to the list in the link below it is), it should appear in the Print option of whichever App you are using to Print.

For instance in the Mail App tapping the twisted Arrow at the top right corner will bring up the action menu, the bottom option is Print. Tap on it to open the Print dialog, and then on Printer - Select Printer to have the iPad look for it if its not already listed.

AirPrint basics

http://support.apple.com/kb/ht4356

You don't.

Either the printer is airprint compatible or they have an app.

http://support.apple.com/kb/ht4356