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.
Remove the protective foil from both sides of all 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.
Not too tight! For all screws and spacers, it's fine to hand-tighten them. You won't need to apply much force: if you feel resistance when the screw is threaded in fully, it's fine to stop. If you keep on turning the screw, too much force will be applied to the acrylic, which will form cracks. Those cracks make for fragile plates and don't look very pleasant.
It's wise to install the OLED cover first, if your kit has one. When you're using an OLED display, you can use the 12mm spacers (the longest ones in your kit). If you're not using an OLED display, you'll likely have some extra smaller spacers (like the 8mm ones) you can use to reduce the space between the cover and the controller.
It's a good idea to remove the protective cover from the display here, if it has one: it'll be more difficult to remove after installing the cover.
To install, first thread an M2 screw through the PCB, and then thread on a spacer from the other end. Do this for both cover mounting holes. Then, align the cover's mounting holes with the spacers, and thread two more M2 screws through them.
Low profile case
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. They are fully symmetrical otherwise.
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.