Locate the area where you intend to mount your microwave. Most microwaves come with a paper template with diagrams illustrating where to cut specific holes, such as the vent hole.
Tape the template under the cabinet or fixture where you intend to mount the microwave. The area to cut the vent hole will be marked on the template. If you don't have a template, proceed to the next step.
Measure the vent area on top of your microwave with a tape measure and mark the dimensions on the underside of the cabinet.
Cut the vent area out of the cabinet with your saw. You can now mount your microwave onto the cabinet or fixture.