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I apologise for the delayed response. You would use either the left or right. The ground goes to the ground on the Arduino and your chosen channel goes to the analog readout of the Arduino.
That question is fairly broad so I hope this reply helps. I wanted to be able to use this with any audio so I found another instructable that broke out frequency, credit given above. That instructable is where I got the voltage splitter and all necessary parts for the audio input. It's not the most accurate for timing but is the best I could come up with for a live display.
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