2 - Cut the copper line on pin 4, like this:
4 - Solder the relays on the outer column of the row of three, starting at IC pins 4 and 5. Red outline. There's no polarity on the relays so orient them any way you want.
5 - Solder the resistors according to the circuit in part I. On resistors R2 and R6, solder one leg and leave the other hanging for now.
6 - Tie all grounds together.
7 - Solder 4 wires to the board: V+ to pin 8, Ground to pin 4, input A to pin 2 and input B to pin 6
8 - Solder one more small cable from ground so we can tie it to the GP's chassis by using the internal screw that holds the jack board. See last picture.
9 - Insert the LM358 OpAmp into the socket and apply power to the board using any old wall-wart power supply with around 12V DC.
10 - Test the circuit by tying the inputs to ground and checking for continuity across the relay's contacts.
11 - If you're reading this, then you didn't get electrocuted. Good.
12 - On the GP, take the cover off and remove the external jack board.
13 - Carefully remove the two jumpers L1 and L2.
14 - Solder one pin of the relay contacts (not the coil) to the jack output part of L1 (and then do L2) and the other contact to ground.
15 - Solder R2 to the resistor part of L1 and R6 to the resistor part of L2.
16 - Get that?
17 - Run a wire from power (pin 8) to the output pin of voltage regulator IC40 on the GP. The output pin is the one on the right if you look at it from the side where you plug your guitar into. Be very careful not to cross any leads with solder and do not overheat the board.
18 - Use the internal screw to tighten the board down and ground it.
19 - Double check all connections before you power up.
This is how it looks on my GP: