You can make a nice battery pack without understanding anything at all about electricity except the difference between + and -, however messing this up could cost you an expensive iPod repair.
Everyone is prone to dyslexia and reading comprehension issues, and trusting experts is a sure way to get hosed, sooner or later. The only safe way to proceed in getting +, - right here is to match what you're building against a working device provided by the original manufacturer.
Once you've cut your FireWire cable in half and stripped the power leads, you can plug one half into the iPod power supply. Next to this, snap a 9V snap connector with wire leads onto your battery holder loaded with AA cells. Take a volt meter (a $12 one is fine here!) and figure out which FireWire wires correspond to your battery leads, giving you the same positive voltage reading when you measure, going back and forth. Now, disconnect the battery holder, and hook up the other half of your FireWire cable to the battery leads as if you were reattaching it to the "live" FireWire cable.
In short, it is far more useful to understand the "scientific method" than to actually understand any science.
You should be highly confident at this point that you got this right, without having to trust any "expert". However, it is possible to get the above backwards. The above check was to save you the time of perhaps having to solder twice trial-and-error, it isn't an acceptable check before actually "going live" and pluggging into the iPod. Here, you have to measure the voltage at the SendStation iPod dock connector, and make sure you get the same +, - reading using your battery pack as using the Apple iPod power supply. This is tricky, you'll have to solder yourself a voltmeter probe extension made out of needles. (I happened to have such a probe already built as part of a pre-9/11 "travel" kit for connecting my modem on Sicilian islands, etc.)
Perhaps it is clear from the above that I don't really trust anybody, least of all myself. This advice may be controversial, orderly engineering society relies somewhat on trusting the advice of our elders. However, I believe that if you're not willing to follow the above steps, you should just buy a battery pack. The Solio is wonderful.
