DPDT Slide switches, back to front

Hi All.   I have already raised this thread. But I cannot trace it, I installed two identical DPDT Slide switches , but, me being the twit that I am, unbeknownst to me, I set one into the baseboard “ the opposite way round “ from the other one, confused? I was, why a short circuit? I had so much help from YMRC but to no avail. Then one day I was on the YouTube channel, and this bloke had a homemade device, with three wires, Red, Black, and Green, and two LED’s,The Green wire was connected to the red LED and vice versa plus crocodile clips.
Trouble is, it works and I sorted out the problem! but I cannot find the notes that I had written down. And now due to a broken wire, I am starting again. And what’s more, I am installing a new programme track, and all this fuss over a four foot long “Shunting Puzzle “ . But it goes something like this? You connect the Red and Green wires via crocodile clips tothe running rails. The third wire you offer it up? To? And depending which LED illuminates? That finds the short.  Job done.  Best wishes Kevin