Mix 1/2 cup of lemon juice with 1 cup of water in a microwave-safe bowl. Place the bowl in the microwave, and heat it for five minutes or until boiling.
Let the bowl sit in the microwave for 15 minutes after heating to allow the steam to loosen caked-on food and grime. Meanwhile, remove the bottom tray, if possible. Wash the tray in a dishwasher or by hand with dish detergent and hot water.
Combine 1/4 cup of baking soda, 1 tablespoon of salt and enough hot water to make a thick paste.
Apply the paste to the walls of the microwave, and scrub gently with a soft-bristled brush until the food and other contaminants are removed.
Wipe the entire surface with a clean, wet cloth to rinse the baking soda solution from the microwave. Dry it with another cloth, and then replace the microwave tray after it is clean.