Due to the similarity of the FoldKB build to the Quefrency/Sinc/KBO-5000, see the build guide for that instead: Quefrency/Sinc/KBO-5000 Build Guide.
A specific build guide for the FoldKB will be provided later here.
For the Rev. 1.0 PCB, a trace is missing on the left 1.5u thumb key for the left half as marked here:
To fix this, after solder all of the switches, add a jumper wire between these two pins as shown below: