Step one: Use your vinegar/water spray and wipe down the whole inside, getting the loose stuff out first.
Step two: Put some baking soda in a bowl and add in just enough water to make a paste out of it. It doesn't take that much water at all.
Step three: Spread the paste all over the inside of the microwave. I used a plastic baggy over my hand to do it because I couldn't find my cleaning gloves and I didn't want the paste to irritate my skin. Let this sit for a minute or so, longer if the mess is really bad, lol.
Step four: Get a wet rag and wipe the paste off, using a bit of pressure and a little bit of elbow grease. It should come up without too much effort.
I just did this and was amazed at how easily all the gunk came off of the top of the microwave! I used the left over paste to make it super easy to clean off the rotating glass circle thing... not sure what that thing is called, haha!