• eth0 is the WAN
• eth1 is the LAN

First, turn on ip forwarding:
echo 1 > /proc/sys/net/ipv4/ip_forward

Next create iptables rules:
/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE /sbin/iptables -A FORWARD -i eth0 -o eth1 -m state –state RELATED,ESTABLISHED -j ACCEPT /sbin/iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT