Install standoffs into the plate.
Assemble the switches and PCB onto the plate, without soldering any switches.
Double-check your layout with keycaps, and verify your LEDs are soldered in good positions.
Solder your switches into the PCB. There are many guides online for this, Adafruit has a good reference guide.
Attach the acrylic cover to the PCB with 4 standoffs and 8 screws.
Attach rubber feet and/or tenting feet to LED PCB.
Screw the LED PCB onto the standoffs installed earlier.
Attach the side pieces to your Pan switchplate.
If you have weights, use the included M3 screws/bolts to attach them to the bent metal piece.
Attach the bottom to the side pieces.