Ubuntu Firewall Box: How to pass packets from Internal Interface to External Interfac

[duplex50]

Hello Again,

Everyone was extremely helpful on my last posts, so I am back again!

My Ubuntu 12 server has eth0 attached to dsl model and eth1 is attached to internal network via a switch. I can now successfully talk internally through eth1 and externally to the internet via eth0.

I would like to know how I can start using eth1 as the default gateway for clients on my network.

eth0 is configured as 192.168.254.2 and eth1 is configured as 192.168.1.23.

I assume that packets are passed from 192.168.1.0 network to 192.168.254.0 network via IPTABLES. I plan on using fwbuilder as my front-end for IPTABLES. Before I start using this front end, I would like to make simple IPTABLES entries via command line so I can point my workstation to 192.168.1.23 as the gateway and connect out to the internet. Once again, I just want to test connecting to the internet from my workstation (192.168.1.246) through eth1 on my server (192.168.1.23) out through eth0 (192.168.254.2) to the internet before implementing the front-end.

So:
Workstation (192.168.1.246) requesting www.google.com -->via--> Firewall eth1 (192.168.1.23) -->via--> eth0 (192.168.254.2) -->via--> DSL modem (192.168.254.254).

What do I have to do to access the internet from my workstation through the Firewall?

Please let me now if any Output would assist.

Thank you all very much again!

[duplex50]

I also want to set this up manually without using front-end GUI for learning purposes as well. I want to understand what all is going no regarding the networking setup.

Thanks.