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
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).