Center punch the two larger marks and drill with a 9/32-inch diameter drill. Center punch the two ends of the area that will be slotted and drill those with a 7/32-inch drill. Cut the area of the slot out with a rotary tool with a reinforced cutting wheel. Place some masking tape along the edges between the holes to guide you. You can do this cutting with the high-speed 3-inch tool, but you will have greater control with the smaller Dremel-type device.