These instructions are on the Issue list for improvement for the final V7.0.0.
There are some photographs which should help in OpenFlexure Microscope v7.0.0-Alpha is nearing , the top post has the boards attached to the converter plate in the draw. Post 11 has the wiring completed. There is no hole provided for wires to supply power to the motors, I cut a small notch in the side of the electronics draw above the Pi USB/network ports and fit the 5v motor power wire in there.
The two-layer base that you have seen may be the base for the Microscope V6.1.5, which is very different from the base for V7.0