Fill a microwaveable bowl with 1-2 cups of water and add 1-2 tablespoons of vinegar. Add a drop of essential oil if you don’t want your microwave to smell like vinegar. Put the bowl in the microwave and shut the door. Microwave the water and vinegar for 5 minutes. If your microwave is REALLY dirty, don’t open the microwave right away when the timer goes off. Leave the door shut for about two more minutes to let the steam continue to work. The steam will help loosen up caked on bits of food, and the vinegar will help eliminate any odors. When you open the door, be careful pulling the bowl out; it will be very hot! Remove the turntable tray (carefully, it will be hot as well) and wash it in the sink. Now just take a cloth or sponge and wipe down the inside of the microwave. The dirt and food will come right off!
Read more here: How To Clean A Microwave With Vinegar And Steam!