Build guide for Free Willy which can be purchased from https://www.squashkb.com/product/free-willy
Start by soldering the SMD diodes on the bottom of the PCB. These have a specific orientation, so make sure the line on the diode (can be hard to see, look closely) lines up with the line on the PCB. To solder SMD components I recommend adding a small amount of solder to one pad, placing the component on the pads, reheating the pad while holding the component in place, then let it sit for a second. Once it cools you can add solder to the other pad.
Next, solder two headers onto the pro micro area of the PCB. You can secure these with electrical tape while soldering if needed. After soldering the 24 header pins, solder the pro micro to the headers. The components of the pro micro should be facing up, and the USB port should be facing out to the left.
Finally, solder your switches and keycaps. When you are finished, you can flash Vial firmware to the pro micro which can be downloaded from here.