Here's a pic from a while back when I added the driver's glovebox to my car:
I first removed the vent, then the light switch assembly, remove the insulation behind the dash where the cutout will be, then created a cardboard template of the hole required to fit the glovebox, marking with a sharpie the horizontal and vertical center-lines of the box. On the dash, lay some masking tape on the dash, marking the center of the area where the cutout should be, then extend the lines to the edges of the area for the glovebox cutout. Then lay the cardboard template on top of this, lining up your centerlines. Then draw the edges of your template onto the masking tape on your dash. Then use a dremel or fein multimaster to make your initial cut 1/4" inside this template cutout, then carefully test fit the box, expanding the cutout hole as needed to fit the box and keep it level. Don't forget that the bezel around the glovebox will hide any minor gaps or the "roughness" of the cut. I can tell you that using a Fein multimaster with a narrow cutting blade is the perfect tool to make this modification, and will make this job quick and effortless. Such a useful tool around the house as well.