How do I clean the inside of the SCX-4200 to fix print quality problems?

Turn the machine off, unplug the power cord, and wait for it to cool down before opening the front cover and removing the toner cartridge. Use a dry lint-free cloth to wipe away dust and spilled toner from the toner cartridge area and cavity, being careful not to touch the transfer roller underneath. Also locate the long strip of glass (LSU) inside the top of the toner cartridge compartment and gently swab it clean. Reinsert the toner cartridge and close the front cover before plugging in and turning on the machine. (Page 8.1)