This is an optional step; you only need to do this if you have purchased a case for your keyboard. There are separate steps for a low profile case and a high profile case.
Low profile case
Remove the protective foil from both sides of the acrylic plates. With some colours of acrylic, the layer will be translucent, so please do check whether any foil is present, as it can be difficult to remove once installed.
There will be a "kb" engraving on the same side on both acrylic plates, so it'll face down on one side, and up on the other.
Align the bottom plate's mounting holes with the spacers, and one by one, put a screw through each mounting hole and screw it down. You won't need to apply much force - hand-tight is tight enough.
Overtightening the screws may damage acrylic. Hand-tight is tight enough.
Lastly, stick the rubber bumpons onto the bottom plate. Place one bumpon on each corner. On larger keyboards, it helps to place one along the top row, and sometimes one along the bottom row or near the thumb cluster as well. You'll have six bumpons to use per keyboard half.