Canon NP6551 Operator's Manual page 173

Align the protrusions of the toner bottle with the
3
slots on the toner supply port. Gently press the
toner bottle down onto the toner supply port until
you hear a click.
MEMO:
●
The toner bottle is now locked onto the toner supply
port.
Press down lightly on the toner bottle with one
4
hand, and pull out the black shutter of the toner
box slowly as far as it will go.
Grip the shutter of the toner bottle, and pull it out
5
slowly until it stops.
Toner flows into the toner box.
Lightly tap the top of the toner bottle to make
6
sure that all of the toner empties into the toner
box.
After the toner bottle shutter has been pushed
7
back in all the way, push the toner box shutter
back in as far as the
5-10
position.