How to replace waveguide cover

After some rather spectacular arcing and finding microwave kaput, realized the waveguide cover had been damaged and needed to be replaced. Ordered a new one from Appliance Repair. Took a while to get it, and, when it finally arrived realized that the old one I had removed (dumb me) had vanished and the new one I was shipped looks MUCH larger than the original. I don’t have a clue which side goes up. Was hoping to find a diagram somewhere, but so far have struck out. Needless to say, family not too pleased with yours truly. Can anyone help? Please?