Install Button Actuators
Insert the button actuators into the button hole cutouts. The base will keep them from falling through. Orientate the case so the button actuators stay in place.
Mount Button PCB
Place the button PCB over the two standoffs that are next to the button actuators. Line up the mounting holes with the standoffs. While holding the button PCB in place, insert and fasten 2x M2.5 x 5mm short machine screws. Fasten until fully tightened.