I fill a pyrex bowl with water and stick it in the microwave and run it at least ten minutes. open the door, wipe down the sides with paper towels. For a nice smell, slice a lemon into the water.

I agree with both of these methods. Any way you can get the inside warm and steamy, that's how the hard crud loosens up and wipes off.

And note OP says "cotton". Some synthetics burst into flame quickly. I really stunk up the place when I left a knitted potholder under a bowl in the microwave.

I agree with all these methods. However, stay by the microwave with the dish towel. If the towel dries up, it WILL catch fire! The bowl is a much safer way. Add some lemon slices or orange slices. They smell delicious and the acid will eat through nasty messes.

I learned yrs ago to just put a large container w/water & one drop of dishwashing liquid in. Turn on for several minutes & then let sit for a few minutes. A paper towel or microfiber cloth will take everything right off. The orange & lemon ideas are great!

After years of battling microwave gunk, I found that what works for me is to mostly fill a glass measuring cup with water, add white vinegar and (optional) a couple of drops of lemon essential oil. Microwave the measuring cup for five minutes and after that let the microwave sit for five minutes unopened. Then just wipe away the condensation! It works perfectly.