Canon Laser Shot LBP-1120 User Manual page 44

When using C5-size envelopes, remove any stiffness by lightly flexing both
edges of the envelope.
2
Insert the envelopes one at a time into the multi-purpose tray, in the direction
shown in the illustration.
If the flap is along the long edge of the envelope, insert each envelope with the
flap at the back and on the right.
If the flap is along the short edge of the envelope, insert envelopes with the flap
at the bottom and at the back.
Printing Envelopes from the Multi-Purpose Tray 2-17