You buy a volume control from Radio Shack, take the pod apart, then cut out the little board or wheel. You'll have 3 wires in there for each half: left, right, and ground. Ground is probably uninsulated twisted copper. You can just connect the ground wires directly. Put a quality 75 ohm resistor in between the left wires and the right wires on each half (so like white from one half -> 75 ohm resistor -> white from the other half). Add some hot glue or electrical tape or something to prevent shorts, stuff it back in the little pod, and close'r up. Done.
Also, if you're not sure you want the S value exactly and you have a multimeter, you can buy a RS volume adapter, dial in the impedance you want, hook up the Etys, and take a listen. Then if you prefer X (25 ohm resistor), for example, you can order quality resistors in that value to permanently mod into the case.
I prefer the sound of the X version -- 20-40 ohm in series to the ER-4P (the picture shows an adapter with 2x 27 ohm).
Additionally I am enthused by a passive EQ network inspired by Siegfried Linkwitz, in the case of my modded Etys reduced to one circuit with the values R0: 20 ohm / L20: 2.25 yH / R20: 15 ohm / C20: 225 nF -- applied to the ER-4P. Still in the prototype phase, but sounds great!