Turn off the power to the bathroom fan at the breaker
Remove the fan cover
Wash the cover with warm soapy water
Dry the cover completely
Vacuum dust from the fan housing and motor area
Wipe accessible parts with a dry or slightly damp cloth
Use a soft brush to loosen stuck dust
Clean the fan blades if accessible
Check for debris in the vent opening
Reinstall the cover
Restore power to the fan
Test the fan to make sure it works properly