Be sure to attach the unit to both the back wall and down through the cupboard where three drill holes are needed, two for attaching bolts, one for electrical feed.
The proper way to install a under counter microwave is to make sure there is at least a 30inch opening under cabinet. Then make sure their is proper backing.
The microwave should have come with mounting instructions. You will likely need assistance since the microwave is heavy. Usually installation of mounting brackets is necessary. Unit then slides on.
I recently had 1952 era cabinets refaced but they come down so far I can't have my microwave on the counter. Looking for a small microwave off the counter. Only about a foot of space under counter. Any suggestions?