96redformula7's write-up is perfectly correct, if a bit unnecessarily complicated.

All that is needed is:
1. Solder a wire on two of the legs on a variable resistor. The other has to be the middle leg.
2. Tune the resistance between the unsoldered ends of the wires to ~500Ω.
3. Solder one wire to mod solder point (red/orange dot in the pic) and the other to a ground (any of the blue dots, eg.).
4. Done.