Marking the polarity requires a test fixture. I use a pair of C batteries, connected together in series by a few resistors, with an LED wired across the hot end of the batteries, right where the test leads attach, to let me know that the “power is on”. it does not matter which side of the LED socket you mark – all that matters is that you ALWAYS mark the same side the same way. I do this by soldering wires to the battery with a clip on one wire and a bare end on the other. When an LED socket is correctly connected, the LED lights up and I mark the side where the clip is attached. Make sure you use something that makes marks that are really easy to see.
Hacking LED Christmas lights for low voltage use
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