If you dont have the user's manual, here you have it online :
Look at page 24 - "Installing the toner cartridge" where you'll find this, but also pictures to illustrate how to do it.:
Grasp the front cover and pull it toward you to open.
Because the printer is very light, it may be moved when you
use it; for example, when opening/closing the tray or installing/
removing the toner cartridge. Be careful not to move the printer.
Remove the toner cartridge from its bag and remove the
paper protecting the cartridge by pulling the packing tape.
Gently roll the cartridge 5 or 6 times to distribute the toner.
Thoroughly rolling the cartridge will assure maximum
copies per cartridge.
If toner gets on your clothing, wipe it off with a dry cloth
and wash clothing in cold water. Hot water sets toner into fabric.
• To prevent damage, do not expose the toner cartridge to light for
more than few minutes. Cover it with a piece of paper, if left
exposed for more than a few minutes.
• Do not touch the green underside of the toner cartridge. Use the
handle on the cartridge to avoid touching this area.
Locate the cartridge slots inside the printer, one on each side.
Unfold the toner cartridge handle and grasp it. Insert the
cartridge in the printer until it snaps into place.
Carefully insert or pull out the toner cartridge not to scratch
against the printer.
Close the front cover. Make sure that the cover is securely
closed. If the cover is not firmly closed, printing errors may occur when you print.
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