Canon IR1600 Reference Manual page 102

3
Push down the metal plate to lock.
4
Turn the opened surface of the paper stack over, and place it in the paper drawer.
Even out the edges of the paper stack. Load the paper stack against the right wall of the paper drawer.
When loading paper in the paper drawer for the first time, set the size plate for the paper being loaded.
When loading paper in the paper drawer, make sure that the paper size setting of the paper drawer
5
matches the size of paper.
5-4 Loading Paper