We recommend using an expression pedal with a 10-50 kΩ linear potentiometer. It should have a 1/4" TRS plug with the pedal-supplied voltage on the ring, ground on the sleeve, and return voltage on the tip.
Here are some examples that we have tested:
Dunlop DVP4 Volume (X) Mini Pedal
M-Audio EX-P (switch set to "M-Audio")
Mission Engineering EP-1
Old Blood Noise Endeavors Expression Slider
Set the switch on the bottom to "STANDARD" and turn the maximum value knob on the right side fully clockwise.
Turn the minimum value knob fully counter-clockwise (0).