Microwaves that have cooked on food sides splattered on all can be very difficult to clean. I have the found the most consistent way to clean microwaves is to have a spray bottle of one teaspoon of Dawn with water. Shake it well then spray thoroughly to all sides of the microwave. If the microwave has a removeable plate, place that soaking in a sink full of hot Dawn and water.
Once the microwave is sprayed down, put the microwave on for 30 seconds to a minute. Then wipe it out with a clean sponge.
Once it’s clean, you can deodorize the oven with a bowl of water and cut lemons. Put that in the oven for about 30 seconds and your microwave will be clean and smell fresh.