I use puppy to share my dialup to the internet. I just have the following two lines in my /etc/rc.d/rc.local:

iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
echo 1 > /proc/sys/net/ipv4/ip_forward

When the system boots up the internet connection is automatically shared. Don’t need to use the firewall setup at all. The only other thing needed is to make sure your LAN network card has an IP address (i.e. Menu > Setup > Network Wizard > eth0: I use local network IP address 192.168.0.1 with netmask 255.255.255.0 and I blank out the gateway address).