You will essentially follow the Middle Box instructions from the Tor web site to make this work.
I used OpenBSD 4.9 for the task, but the instructions show many operating systems will work. So, after a clean install, make the torrc, pf.conf. and dhcpd.conf changes. Use static addresses or DHCP at your own discretion.. This config assumes inteface pcn0 to be the gateway and pcn1 is the dhcpd LAN and they are already configured.
Configure torrc with transparent mode settings as described in the Middle Box guide.
Configure Internal Lan as 192.168.168.55.10.
Configure dhcpd.conf to serve that LAN.
Configure rc.local to start Tor on bootup..
Many will use this for free public Wi-Fi through Tor. I use it as the basis of my Tor test network. I setup an internal interface and I have Damn Small Linux virtual machine boot up and get a DHCP assignment from the Tor middlebox. I have said before that Tor can do so much more than just be an anonymizing tool.