Do you want internal or external control?
For internal control, you'll want to find a trim pot. 500 ohms. Connect one pad for the resistor to the middle leg, and then the other pad to either one of the outside legs. Doesn't matter which.
For external control, get a 16mm Alpha B500R pot, and wire one pad for the resistor to the middle lug (lug 2). Then wire the other pad to lug 3, the right side lug if the lugs are towards you and the shaft is pointing up.
Jacob