Brother MFC-8440 User Manual page 189

4
Gently rock the toner cartridge from side to side five or six times
to distribute the toner evenly inside the cartridge.
5
Remove the protective cover.
6
Insert the new toner cartridge in the drum unit until it clicks into
place. If you put it in properly, the lock lever will lift automatically.
Toner Cartridge
12 - 30 TROUBLESHOOTING AND ROUTINE MAINTENANCE
Drum Unit