Note: Type all the following commands in a root terminal, DO NOT use sudo.
1. Start by configuring the network card that interfaces to the other computers on you network:
# ifconfig ethX ip
where ethX is the network card and ip is your desired server ip address (Usually 192.168.0.1 is used)
2. Then configure the NAT as follows:
# iptables -t nat -A POSTROUTING -o ethX -j MASQUERADE
where ethX is the network card that the Internet is coming from
# echo 1 > /proc/sys/net/ipv4/ip_forward
3. Install dnsmasq and ipmasq using apt-get:
# apt-get install dnsmasq ipmasq
4. Restart dnsmasq:
# /etc/init.d/dnsmasq restart
5. Reconfigure ipmasq to start after networking has been started:
# dpkg-reconfigure ipmasq
6. Repeat steps 1 and 2.
7. Reboot. (Optional)
I hope this helps.