Run power cable from battery (make sure inline fuse is no more than 6 inches of wire away from the battery post), through the firewall (there is usually an unused grommet or two, or another cable which has enough room to run another wire.
VOILA! Put the respective wires into the PSU with terminal connectors. I don't know if these PSUs come with protectors but if you can get your hands on something to keep stuff like metal from touch or falling on the terminals, that would be nice. Just from experience hooking up audio.