Make sure your computer is new enough to boot off of firewire. I think G4's could, i don't really know when the cuttoff was.
Boot off of the install CD by holding C. Once the installer comes up open disk utility from the installer menu. Start your firewire drive up. DU should recognize it. If you need to format it, do that now, otherwise just make sure it is there. Then exit DU and follow the installer, choosing the firewire disk as the install target.