Jack, the first thing that I can tell you is that you can't be too scared to get started. However, have a little fear is never a bad thing when you are messing with an expensive piece of equipment.
Having done this mod just a few weeks ago myself, I can tell you that it was a piece of cake.
First you will want to start out with a pair of cathodes. Gearbox has some great stuff for excelent prices. I would choose the 12" Cathodes.http://stores.ebay.com/computer-parts-wholesale-lots
They sell the pair that has everything you need to complete the mod. It comes with:
(2) 12" Cold Cathodes
PCI switch to turn the lights on and off
Inverter that can control both cold cathode lights
Mounting Tape, Velcro Tape, Screw included [/ul]
Personally, if I were doing this mod again I would get a set of smaller cathodes as well. It is very difficult to get the cathodes to emit light through all of the sections of the G5. I had to do some dremel work on my system to get a 12" cathode in the top. I had never seen anyone do it that way and I felt that it was a much needed addition. The G5, as a result, looks alot more equalized.
You can have a look at this thread where I submitted some images.http://macmod.com/component/option,com_simpleboard/Itemid,/func,view/catid,23/id,3077/#3077
Now, you had questions on how to locate the plug. What you are going to need to do is borrow the one from the optical drive. The kit that I recommended comes with the converter. Therefore, all you have to do is remove the optical drive from it's bay. Unplug the power cable and run the splitter. From there you will just run the included wiring to the cathodes and place there where you like. There is a pci switch that you can add to the back of your system so that you can turn the cathodes on and off as well.
The hardest part about this mod is the placement of the cathodes. I am also very meticulous so I spent alot of time concealing the wires. I wrapped them all in electrical tape.
You can see here where I placed the inverter box. It is located on the back of the internal speaker.
Here are some additional picture of where I placed the cathodes in the system. In this first one you can see where I had to dremel out a notch so that the cathode could fit. There is no other way because the cathode is too long. I know that it is not the most attractive cut, but the only way to get that is to take out that fan, and that would require a disassemby of nearly the entire G5. I wasn't about to do that :side:
This next image is a picture of the one that I placed in the front of the G5. What you can not see in this image is the electrical tape that I placed on the revese side, the side that faces the front of the case, you. I know, you are probably asking, why would you do that. Well, all too often these cathodes mods come out and the only thing you can see is the cathode, not the light from the cathode (I know you see the light). I wanted to block the direct light so that it wasn't overbearing. The result, is a pleasing illumination of the case and not a shining light.
On this cathode that is placed in the front, you will have to slide it down the tightly fitting front, between the cheese grated metal and the support for the bottom section. After you have decided your positioning, just place the velcro on the cathodes and put them into place. Keep in mind that once you stick the velcro to the computer its going to stay there. So, don't make any mistakes.
Finally, have fun and be creative, take plenty of pics and enjoy. Let me know if you have any other questions.