Smoothwall installation is incredibly straight-forward, much like any other operating system install. You'll need to have your BIOS configured to boot from a CD, and then installation will automatically begin. The hard drive will need to be formatted so depending on the size of hard drive installed in your firewall machine, this could take several minutes.
After automatic installation is completed, Smoothwall will scan the computer and identify all network interfaces. Smoothwall uses a neat color coding system to identify the purposes of each NIC. Red is assigned to the Internet network interface. Orange is assigned for special purpose interfaces, such as a DMZ (demilitarized zone, term for a more publicly accessible area of the network), and Green for "trusted network" - basically your Internal network, should you choose to set it up that way.
You're going to want your Green interface setup with a static IP address consistent with the addressing scheme of your LAN, such as 192.168.1.1. After you've configured interfaces you'll have to reboot the machine. After the reboot, you'll be able to use the web interface to manage the firewall and router.
You can access the Smoothwall router from your Internal network via your <green interface IP>:44. IE: 192.168.1.1.:441. From there, you'll be able to configure lots of personal options using the Admin username and password you select at Installation.