Summarize this article:

Most over the range microwaves include a short power cord that is designed to feed upwards into the cabinet that the appliance will sit underneath. Ideally, there will be an electrical outlet inside this cabinet that you can plug the power cord into.  Your microwave’s installation instructions will specify whether there are acceptable alternative locations for the electrical outlet. If there is not an outlet in a convenient location, contact an electrician for assistance installing one before you put the microwave in place. In general the bottom of the microwave should be no higher than 54 inches (140 cm) from the floor. In addition, there should be approximately 20 inches (51 cm) to 24 inches (61 cm) between the bottom of the microwave and the top of the range. This will allow you to use both the microwave and the range safely and comfortably.  Use a tape measure to verify the height of your microwave. Measure from the bottom of the cabinet you will install the microwave below to the height of the microwave. Make a mark at this spot. Measure the distance between this mark and the floor, and the top of your range, to make sure that the microwave will be situated within the acceptable height range. The microwave itself will have a fan, and since it sits above the range (where a range hood would normally go), both appliances will have to ventilate outside together. Ordinarily, there are two options: either the appliances will ventilate horizontally through a tube in the wall, or vertically, first through the cabinet above the microwave and then the ceiling or higher up on the wall.  Your microwave may have additional fittings which you should set in place in order to allow it to ventilate properly. Check the installation instructions that come with your particular model to be sure. If you do not already have a ventilation system in place, contact a contractor to have one installed. If your kitchen cabinetry and framing are sturdy, they may be sufficient to support a relatively heavy over the range microwave. If the materials are less robust, reframe the area where you will install the microwave with two by fours. Some experts recommend reframing in all cases.  Your microwave should come with a kit that has a framing template. Place it where you want the microwave to sit, and follow the guidelines to cut through the drywall. Remove the drywall and pull out the insulation from the area inside the wall. Secure two by fours between the wall studs. Check the instructions provided with your microwave for the exact dimensions. Fill in any empty space with new insulation.

Summary:
Make sure there’s an outlet nearby. Check to make sure your microwave can be installed at the proper height. Determine the ventilation direction. Reframe the opening, if necessary.