It looks pretty easy. Probably one of the more ideal mods to get you started in to the modding world. You'll need a simple spst ("on-off") switch (which will have two soldering points the switch) to connect/disconecct JN1 to make it switch between US-JP settings. The "cable" he indicates will basically be nothing more than 2 wires -- each wire connecting to one of the end points of JN1 then other leading to your switch.
Some other things you might consider -- I don't know too much about the NGCD, so enlighten me...
- Now is there even a reason to consider a EU switch? If there is, you can add 2 more wires, and have a 3-way ("on-off-on") DPDT switch, which will have 6 soldering points (but 2 points would go unused). Switching to one end would be US, other end would be EU, and middle would be JP.
- And 50/60Hz switch is another consideration only if you want a EU switch. And even then, you may still not need it. But before I get too deep into reasons for even considering it, let me know first if there would be a reason to add a EU position to the region switch.
And the easiest option to consider -- you can just bridge the endpoints of JN1 together with a single piece of wire, and that would just permanently switch your NGCD to a US region system...