Take a microwave-safe bowl and pour in white vinegar and water in a 1:1 ratio.
Put it in the microwave and heat on high for 3 to 5 minutes until the water boils.
Don't rush to open the door. Let it sit for 5 to 10 minutes to allow the vinegar steam to fully soften the grease and food residue on the inner walls.
Take out the bowl (be careful of the heat), and wipe it from the inner wall to the turntable in sequence with a damp cloth or sponge; most stains can be wiped off easily.
For stubborn stains, you can dip a cloth in vinegar water and wipe them directly.
Finally, wipe the surface with a clean damp cloth to ensure no vinegar residue is left behind.