Canon LBP7780CX User Manual page 159

Slide the rear paper guide.
(1) Hold the lock release lever.
(2) Slide the paper guide to the position of the size of the paper to be loaded.
Align the position of (A) with the size mark of the paper to be loaded.
3
Load the paper with the printing side face-up, and the paper stack aligned with the rear paper guide.
NOTE
When loading paper with a letterhead or logo
See "Paper Loading
4
Orientation" and load the paper in the correct orientation.