Select print - and select printer properties.
Select media = envelope - it should select the rear tray as the paper source.
Put your envelope into the rear tray - closing the jaws so it's centered.
Before printing check that when viewed from a position in front of the printer -
1. the address side of the envelope is visible (facing toward you).
2. the envelope is rotated 90 degrees to the left (e.g. flap is on your left hand side, tucked behind envelope).
Print the envelope.