(C5 is the envelope size used for A5 - i.e. A4 folded in half once - paper)
There are changes to be made in both Microsoft Word and Printing Preferences.
The envelope gets inserted into the SSF (single sheet feed) slot built into the paper tray at the front of the printer. Open the envelope flap, and position the envelope so that the flap is on your right (as you look at the front of the printer) and the “front” of the envelope (where you want the address to be printed) is facing the floor: Example.