The waveguide cover is the component in a microwave that helps convey the electromagnetic waves into the central heating area. It is fairly common for the waveguide to burn out because of grease or other moisture escaping from the food and contacting it. Sometimes a waveguide burns out with no cause. Making a new wave guide cover is easy, if you approach the task correctly.
Buy a piece of waveguide material roughly 8 inches by 8 inches. Most appliance repair stores will carry several sheets of the material and can cut the sheets to fit the size you need.
Bring your old waveguide cover with you, if you can. Buy a piece that is at least an inch larger on all sides. This will ensure that you have enough material to work with.
Clear out a work area on top of a workbench or table. Lay a piece of cardboard on top of the work area. Set the waveguide material in the center of the cardboard sheet and lay your old waveguide on top of it. Position the old waveguide so that the new sheet expands beyond it on all sides.
You may have to make a guess or two about the shape if the old waveguide is significantly burned. Absolute accuracy is not required, though, so just get it as close as you can.
Trace along the edges of the old waveguide using a razor knife, cutting a shallow outline of the old waveguide. Set the knife to the side and compare the outline to the old waveguide to make sure that they are close in shape and size.
Use a small Phillips screwdriver or a punch to make any necessary holes in the waveguide. Be careful when punching holes along an edge. You will have to start over if the material cracks or separates.
Use a strong pair of scissors to cut along the razor knife line. Use the razor knife to cut a slightly deeper line, if necessary.
Compare the two waveguides one last time when you are finished. Place the new waveguide in place of the old one, and you are done.