Set a Bowl
Fill your microwave-safe bowl 3/4 with water. Add two tablespoons of baking soda and place in the microwave. Put a non-metal object such as the wooden cooking spoon suggested above in the bowl, this will aid in the water remaining stable while hot.
Heat and Wait
With the spoon inserted in the microwave-safe bowl, set to high for 3-5 minutes and run the microwave. Once the time has elapsed, let the bowl sit in the microwave for 5-10 minutes to allow the steam and heat to work.
Open the door and carefully remove the bowl (as it may be hot) from the microwave.
Using a clean sponge or dish cloth, dip in white vinegar and wring. Thoroughly wipe down the entire interior surface of the microwave with this solution. If the smell of vinegar is too much for you, mix 1 part water and 1 part vinegar and use in the same manner.
Once you have wiped everything down, set the microwave on high again, this time for 3 minutes. Leave the microwave door closed for a few minutes to allow the heat and steam to break gunk down.
Run your sponge or wash cloth under cold water and wring out well. Wipe down the inside of your microwave with this one last time and it should be spic, span and ready for action.