Installing a second car battery is a fairly simple procedure. All you need is two wires, and a battery. Regardless of what you have heard from morons who think they know everything about electricity when it comes to car audio, all you need is some really cheap plain-jane copper wire. Anything from 12 gauge on up is probably adequate thickness since copper is an excellent conductor. The battery size is up to you, just make sure the standard voltage is 12V.
The best location for an extra battery is in the trunk, since there is probably no room in the engine bay without heavy modifications, and you probably don't want a big heavy battery sitting in the cabin of the vehicle.
If you have a car audio amplifier installed, the installation is that much easier. All you need to do is ground the grounding wire from the negative (-) terminal on the battery to the same place you grounded the amplifier. Then run the power cable up to your other battery and attach it to the positive terminal (+) or just attach it to the power cable to your amp where it meets your amplifier. The hardcore attach the power cable right to the alternator, but I have it attached to my amplifier and i have had no problems with power drains, even using a 12 gauge. cable with an 800W amplifier. However you do it, it will work and you will now have at least twice the stored power available that you once had before.
This takes a huge load off of the alternator, extending its lifetime, and allows you to install more audio amps or other electrical devices. No more headlight dimming or crummy sound from the subwoofers. Enjoy your work! :)
The above picture shows my second battery in the trunk of my car. You can see the place where it has been grounded and the wire is being held down by a large washer.