Brother HL-3400CN Series User Manual page 38

USER'S GUIDE
Fig. 2-12 Rocking Cartridges and Removing the Protective Cover
2–8
3. After rocking each of the cartridges 3 to 4 times from side to side,
remove the orange Protective Cover from the Toner Cartridges.
4. Install the 4 toner cartridges by positioning them in the correct guides.
Each color cartridge is individually keyed to prevent improper
installation. Match each cartridge color label to the corresponding
color label on the printer.
Fig. 2-13 Install the Toner Cartridges
5. Close the Front Cover.