OK, I've had a chance to pull together the following parts list:
https://www.sparkfun.com/wish_lists/116237
(Full disclosure: I'm a SparkFun employee. I designed those little PCBs, and when they sell, it shows the folks in charge that audio-related components are worth selling.)
I'm assuming that you've got soldering & drilling tools and insulated wire (say 16 or 18 Ga stranded).
It goes together like this:
1. Drill 4 holes in the chassis. 2 for the TRS jacks, 2 for the switches (3/8" and 1/2", respectively, if I remember correctly.) Consider your hole placement before you drill, so the components don't touch each other inside.
2. Solder the TRS jacks to the PCBs. They go on the side with the jack traced in outline. You can snap the little flanges off the PCBs, if you want.
2.a. Solder a piece of wire to the GND terminal of the PCB for the Input jack.
3. Mount the jacks and switches in the holes.
3.B. Take the wire on the GND terminal, and loop it around the threads of one of the switches, so that then the switch is tightened, the wire touches the chassis.
4. With a piece of wire, connect the S pad of one jack to the S of the other.
5. With a piece of wire, connect the R of one jack to one side of the switch to control the Right earpiece. From the other side of the switch, go to the R of the other jack.
6. With a piece of wire, connect the T of one jack to one side of the switch to control the Left earpiece. From the other side of the switch, go to the T of the other jack.
7. Test it out.
8. When you're confident it's working, put the back on the enclosure.