Things to Consider
Detailed installation instructions should be packaged with your microwave. Make sure yours are provided, as they will contain the diagram for your specific machine.
Make sure you know local code for this job and comply with it. Ideally you will want a 30" cabinet above your range. An over-the-range microwave is normally 15 inches to 16 inches tall and most building codes for new construction (check your local codes) require 18 inches to 20 inches of space between the range and the microwave. You will want at least 30 inches between cabinet and stove with 34 inches being preferred by some installers.
You will need an electrical outlet inside the upper cabinet. If there is an existing microwave the outlet will already be installed. If there is a hood above your range, you will need to install an outlet, as hoods are wired directly like a light fixture. When removing an existing hood, you will need to go turn off the power so you can move the wires into the cabinet to install an outlet. The following instructions are for hanging a microwave to an upper cabinet with an outlet.
Ducted or Recirculating
Decide if you want your microwave to be vented to the outside or recirculated. If you are venting the microwave, you will need a 3 1/4 inch by 10 inch duct centered out the back wall or up into the cabinet to tie into your stove venting. A recirculated microwave recirculates air through a charcoal filter and requires no venting. Most microwave/hoods are now convertible.
Hanging the Microwave
First remove the mounting plate from the back of the microwave by removing the two screws. Pull the back off and remove the blower to turn it to the proper position for vent or recirculation. Instructions for this will be with the unit or available from the manufacturer. Drill a 1.75 inch hole in bottom right hand side of cabinet for the microwave wire to go through.
Your microwave instructions will include a template to help you mark proper placement when installing the hanging bracket to the wall. Try to find studs to attach the bracket to, or use the drywall anchors provided. Attach the microwave to the bracket and run the wires and/or ducting up inside cabinet, then position into place centered in your opening.
Consult the template, and install the screws provided inside of the upper cabinet. When tightened, these screws will basically suck the microwave up against the upper cabinet. This is an important step, as the long screws that go down through the front of cabinet into the front top of microwave provide the mechanism that holds your microwave in proper place. The purpose of the wall brackets is simply to keep it from falling.
Finally, plug in your microwave and check to make sure it is functioning,