Step 1: Solder in the pot
Take the 50k potentiometer (pot) and attach 40mm lengths of wire to each of the legs.
Using a craft knife, carefully cut the trace between R1 and R2 (the 10k resistors at the line socket) and C1 (the electrolytic capacitor).
This image shows the underside of the PCB.
Solder the wire from leg 1 of the pot to the board at the pad where R1 and R2 join. Solder the wire from leg 2 to the negative of C1. Solder the wire from leg 3 to the ground.