Canon BJ-20 User Manual page 43

Press the LF/FF switch.
3
The paper advances to the position ready for printing.
Rotate the paper thickness adjustment dial until you feel a click.
4
For envelopes, transparencies, and thick paper:
Rotate the dial as far toward the back of the printer as possible.
For high-density prints (e.g. graphics) on standard paper:
Rotate the dial to its center position.
For normal print on standard paper:
Keep the dial as far toward the front of the printer.