Remove the rotating tray from the bottom of your microwave and place it in equal parts hot water and white vinegar to soak.
Fill a clean, empty spray bottle with undiluted white vinegar and spray the inner surfaces of the microwave. Allow the vinegar to soak into the stains for 15 to 20 minutes.
Moisten a cleaning sponge and dip it into a few tablespoons of baking soda. Use the sponge to wipe out the microwave and to scrub the tray that you left soaking.
Wipe out the microwave once more with a cloth that you dampened with clean water.
Wipe the microwave dry with a clean, dry cloth.
Rinse the rotating tray and dry it before returning it to the microwave.