Step 5: Replace the potentiometer
The next thing to do is solder short wires to the holes where the leads of the old potentiometer used to be. This should take 8 wires.
Next, you should make any necessary modifications to the faceplate. I drilled two large holes and two small locating holes for the potentiometers, as well as cut out a hole for the new power switch.
Test fit the potentiometers, and cut the shaft to length using a small saw.
Now, making sure to keep the same orientation, solder the other ends of the wires to the new potentiometers.
Then, solder the two wires left over to the switch. You may have to put the switch wires through the switch's hole before soldering. Wrap the leads in electrical tape to prevent shorts.
After this, I plugged in the wall-wart transformer and powered up the circuit to test. The LED power indicator lit, so I plugged in a pair of headphones and listened for changes in sound when I turned the potentiometers. This is how I figured out which Potentiometer controlled left and which controlled right sound channels.