Canon imageRUNNER C3489 Reference Manual page 353

7
Press down on the envelope receptacle, and move the front
width guide to align it with the mark for the desired envelope
size.
8
Fix the front width guide in place with the screws.
9
Move the back width guide of the envelope feeder attachment
in accordance with steps 6 to 8.
Envelope Feeder Attachment-C2 (Optional)
7
7-23