You have to figure out the matrix before you cut it. Then repair those traces that are still needed (here the black wires on the main pcb):
As you can see, only a few repairs were necessary on this Cherry pcb.
Before I cut it, I drew a line where the cut was going to be. Then added row/column numbers to the traces that crossed this line and would be cut.
New controller in the lower right.
Safety warning: pcbs contain Formaldehyde. Do the cutting outside, wear a respirator. Don't breathe the dust.