It depends on what you are plugging your coil/cord INTO. There are at least two or three different "standards" for how 3.5mm connectors are wired. And they are not compatible.
Audio recorders, consumer camcorders, DSLRs, et.al. use the sleeve for ground and tip for left, and ring for right. You would wire one side of your coil to the sleeve (the "ground" side), and the other side to BOTH tip and ring (to send the signal into stereo Left + Right channels.)
Computer microphone inputs (the kind with the pink jack) again use the sleeve for ground, and the tip for the (monaural) signal. But they apply ~5V to the ring for power for the (typically) electret condenser microphone. If you were connecting to this kind of device, you would want to connect your coil to sleeve and tip and leave the ring unconnected.
Smart phones, and some newer computers use 4-terminal TRRS plugs. The tip and first ring are the left and right signals to the earbuds. The second ring is typically the ground connection, and the sleeve is typically the microphone input. If you were connecting to a phone then you would want to wire your coil between the second ring and the sleeve and leave the tip and first ring unconnected.
If you chop into the length of your pre-wired cable, you won't really know which color wire goes to what part of the connector. Recommended to test each wire with even a cheap $5 DMM or a flashlight battery and bulb as a simple continuity checker.
Stackexchange seems schizophrenic about questions. You never know when (or WHY) people "down-vote" questions. And the rules still seem odd and inexplicable to me.