After many rounds of testing (and soldering), I finally got the potties to work.
First up: volume control. Going clockwise, the volume decreases. Going anticlockwise, the volume increases. It’s the most obvious out of the two.
Next: gain control. This might be hard to tell.
Potentiometers are technically called variable resistors — you can control the amount of resistance that flows in a circuit when you turn the knob in one direction. When there’s more resistance, less current will flow, and vice versa.
Going clockwise, the resistance decreases. Less resistance equals more gain, and more gain equals more distortion!