The assembly guide is currently being written, but assembly should not be too different from most electronics projects. A few things to note:
This is an intermediate level project. You should have some experience with surface mount.
It is recommended to socket all DIP ICs. Double-socketing the sound chips can also be handy for chip swapping.
Do not populate RN2 until the ESP32's 3.3v flash setting efuse has been burned. Instructions regarding this can be found in the flashing guide on the main MegaGRRL repo.
Populate the SD card slot before the heatsink or USB connector. The positioning of these parts can make the SD card slot difficult to mount.
The ground pad on the bottom of the ESP32 does not need to be soldered.
Screw the 7805 to the heatsink before soldering either part. This will ensure the regulator is spaced the proper height above the PCB.
The 3.3v regulator can either be soldered or bolted down. Soldering it down may need a powerful iron. Make sure not to keep it too hot for too long, because it could be damaged.
DIP switches should all be on for YM2612, all off for YM3438. This is also listed on the PCB silkscreen.
I recommend first powering the board off a bench supply, bringing the voltage up slowly, watching the regulator output voltages with a meter. Test points are available on the bottom of the PCB for this.
The last parts populated should be the YM2612 and SN76489. This should be done after firmware has been flashed.
For 3D printing the baseplate, 100% infill is recommended. Supports are also needed for the recessed screw holes. The OpenSCAD file uses mils, you will need to resize the resulting STL to 2.54% to print in standard mm.