All the modification happens on the left driver. Remove the left ear pad and this should reveal four screws.

Remove all of them, and now you should be able to see the wires to the driver unit.

Since we just need to replace the long cable with a mount, we won't touch the wires coming from the other driver. Just desolder the three wires which comes from the 3.5mm cable and then pull the cable out. Notice there is a little rubber tube at where the cable come out of the headphone. Remove that too.

Then, pick some fresh wires and solder them onto the solder joints we just desoldered. If possible, use wires of original color since it would make life much easier.

Now, grab a 3.5mm mount. My mount has three ends: one longer lead for the ground and two for both channels. Figure out the order by plugging in a 3.5mm cable and use a multimeter to test the connections. The diagram for a 3.5mm plug can be found easily online.

After figuring out the lead order, put a head shrink on the cable and solder the wires to the connector so that they won't touch each other by accident. Then solder the wires to the corresbonding leads.

Now we can try to fit the connector into the hole. Unfortunately the connector is a bit wider than the original cable, so a drill is needed to enlarge the hole. Gradually use larger drills and eventually the connector should be able to fit in the hole without any effort.

Also, there is a little piece of plastic that guides the cable on the outer side of the hole. It's quite small so I just use a file to remove it. In order to make sure the nut on the connector have enough space, I almost entirely level that surface.

Now we can try to screw the nut onto the connector. Should be really easy.

As you can see, I screw up a little bit and accidentally scratch off some surface on the headphone. No huge deal, though.