The first step before installing your over-the-range microwave, is to install the vent through the wall or cabinet, if indeed you are venting the fumes outside. Your new microwave oven comes with an installation template that will help you mark the appropriate spot on the wall or the bottom of the cabinet to make a hole for the vent.
In the case of the wall installation, mark the wall, cut the drywall out using a drywall saw, and remove any wall studs that interfere with the vent hole. Use a small drill bit to drill through the outside wall so you'll have the four corners marked before going outside to make the cut in the wall.
In the case of venting up through the cabinet, soffit and into the attic, simply use the template and mark the underside of the cabinet to make the cut. In many cases, where an old microwave is being replaced, the new microwave will line up with the existing hole. If not, you may need to modify the hole opening either forward or back to accomodate the new unit.