Lug 3 to switched tip, Lug 2 to switched ring, Lug 1 to switched sleeve
normal TRS (stereo) cable
The switched jack lets you use the pot when no cable is plugged into the exp jack, and disconnects it when a cable is plugged in.
The important thing is to use an insulated jack, and you do not connect any lug of the jack to ground.
I made this for you. Since the pedal in question connects lug 2 of the pot to the output jack, you are going to connect the ring of the exp jack to the tip of the output jack, in this picture the exp jack ring lug is labelled "2".
Basically you are going to pretend the non-switched TRS lugs are 321 for the pot respectively, then the pot itself simply connects to the switched lugs on the jack