There is an 8 pin set of holes on the main board lower right – just to
the right of the screw hole to mount the board onto the enclosure (and
to the right of a print that says “ext out”… Is that what I use? Do I
just solder that in?
Yes, that’s the pin header that you should solder.
I would think the colored wires would go symmetrically leftmost pins (white) on main board to leftmost pins on the expansion board. That means flipping the connector on the expansion board and to do that I need to cut off the little bump…Is that correct? Can I do damage by trying or it just won’t work if the wiring is wrong?
No, you shouldn’t flip the connector: the bump should always face up, on both the controller side and the expansion board side. If you put the two connectors back to back, the wires should all be parallel. This type of cable is the easiest to source, so the expansion board pin header is designed to match the cable wiring. In short the expansion board pin header is ‘mirrored’ from the controller side pin header. Hope this makes sense.