Unplug the microwave
Remove the turntable and roller ring
Wash the turntable and roller ring with warm soapy water
Wipe loose crumbs from the interior
Fill a microwave-safe bowl with water and lemon juice or vinegar
Heat the bowl for 3 to 5 minutes
Leave the door closed for a few minutes
Remove the bowl carefully
Wipe the interior with a soft cloth or sponge
Clean the ceiling, walls, door, and door seal
Wipe the control panel with a damp cloth
Dry all surfaces with a clean towel
Reassemble the turntable and roller ring
Plug the microwave back in