Before beginning to install an over the range microwave oven, ensure your that the bottom edge of the cabinet is 30 inches away from the stove top. If you have a range hood, and most will, there will be an electrical coil. Cut the electricity to the range hood from the breaker box. Ensure no electricity is getting to the unit by attempting to turn it on. If it's off, you can go ahead and take the hood off.
You should utilize a traditional single outlet junction box. Determine where you are going to put the junction box, usually in the back in the right lower corner, trace the box, then drill a hole in each corner of the tracing, and end by cutting out the hole. Fish the coil through your new opening and into the junction box. Fasten the box to the cabinet and put in an outlet. Determine if you're getting electricity once you turn it back on.
Take the wall hanger sheet off of the rear of the microwave and remeasure the 30 inches from the cabinet to the range. Find the middle and draw a level line through it. Fasten the hanger sheet to the wall utilizing either a stud or toggle bolts for sturdiness. Then you need to put in a ventilation system to vent the exhaust outdoors, or install a recirculation fan in the unit itself.
Determine the distance of the screw holes on the microwave and mark them on the cabinet bottom. You will need to include wooden spacers if the cabinet is indented. Make holes where your marks are. To make room for the plug, make a 1 and 1/2 inch opening in the rear right corner of the cabinet. Find a partner to assist you in setting the microwave on the hanger sheet as you will need someone to keep it in place while you tighten the screws. Place the cord through the plug opening.