Find a large, sturdy measuring cup or bowl. Be sure that the bowl you are going to use is microwave-safe. Otherwise it can melt and you'll have a bigger mess to clean up then just food splatters.
Fill the bowl up with approximately 2 cups of water. Do not add any cleaners to the water.
Place the bowl inside of the microwave for 10 minutes, allowing the steam to penetrate the food that has attached itself to your microwave.
Leave the bowl in the microwave, keeping the microwave door closed, and let the steam work on the dried food residue for about half an hour.
Open the microwave door after 30 minutes have passed. Remove the bowl with potholders -- it may still be hot.
Wipe the microwave clean with a washcloth, while the food residue is moist and softened by the steam.
Repeat the steaming process if the food hardens back up before you have finished cleaning it.