Install stabilizers, rotary encoders, or OLED panels.
OLED panels must be soldered flush with the pcb or as close as possible. Trim the header pins and do a fit check before moving forward.
Assemble the switches and PCB onto the case/plate, without soldering any switches.
Install some keycaps to make sure your layout works (specifically the areas with multiple layout options).
Make sure the switches are pushed completely into the PCB, and that the PCB sits properly inside the case. If you have a CNC case, screw on the LED PCB and push until it sits flush with the bottom.
Solder in the switches. There are many guides online for this, Adafruit has a good reference guide.
If you are soldering your own LEDs, you can follow the Zen guide for that step.
Configure the DIP switches according to the guide on the LED PCB, shown below. The default firmware has left as the master and right as the slave.
If you wish to use a custom firmware (like Kageurufu's WIP keymap) that supports per-key animations on both halves, set switches 1-2-3 on the master to Off-On-On and on the slave to On-Off-Off.
Attach rubber feet and/or tenting feet to LED PCB.
Screw in standoffs for the plate case, and install the LED PCB.
Low profile soldering notes: You will need to bridge some pads, specifically for the H, Y, and P keys. Backspace is missing the holes, so you need to either bend the pins, or turn the key 180deg and solder some jumper wires to the pins. Sorry about that.