Tomix LED Board: Where Do I Start?

[ATShinkansen]

Hello,
I am still on my Tomix RailStar 700 right now.  Last week, I managed to kill one FL12 due to a short circuit, which I believe came from the LED board.  I had forgotten about the need to modify the circuit board; I did follow the traces, though, and falsely assumed that the two circuits were independent of each other (one on each side of the board).  I now know that they are in fact electrically connected through the soldering pads somehow.  I verified this by checking the board in the unaltered cab car.  I have cut traces on circuit boards before, so I’m comfortable doing it, but I’m not sure exactly where on the board to cut.