Width: Use measuring tape to measure from the left inside cabinet edge to the right inside cabinet edge. Keep in mind that the oven should be a fairly tight fit in the cutout.
Depth: Measure from the back of the opening to the front of the cabinets. Keep in mind the oven door will extend past the cabinets. Make sure there is at least 20 inches of open space in front of the oven so you can fully open the door to load and unload your food.
Height: Measure from the top of the cabinet opening to the bottom of the cabinet opening. Again, a tighter fit is ideal.
Note: Always reference the manufacturer's cut-out dimensions for your model to ensure correct installation.