ok, See if this helps. I'm going to make some assumptions.
1. you have 2 Ethernet cards, I will call them eth0 & eth1. eth0 will be outside interface and eth1 will be inside interface (outside being public net, inside private net).
2. your internet provider gives you a public address with DHCP.
3. your firewall/router will be the DHCP server for your PCs on your private network.
ok here we go
eth0 will have to be setup to get an address from your provider, if your provider gives that to you though DHCP you will have to have dhcpcd running on eth0 to get the address.
eth1 needs to be static address with a private network address, I would set it up for 192.168.x.1 with a mask of 255.255.255.0, x= any address from 1 to 255. eth1 will also be the gateway address from your private network. all of your other hosts will have to be in the same /24 address space, I would run dhcpd to assign addresses to the rest of your network. I would also configure dhcpd to assign eth1 as the default gateway and assign your providers DNS servers addresses to your local hosts as well. you can find out HowTo
set up dhcpd.conf.
Last Shorewall has to be setup. I'm not going to go into that, the wizards will set it up for you. But if you want to do it yourself, go to www.shorewall.net.
click on the quick start guide. Shorewall site will tell you to remove shorewall that mandrake installed and to reinstall the package from the shorewall site. I DID NOT DO THIS AND IT WORKS FINE, I have manually modified the rules file, but it is not necessary.
Now having said all that, I would use Mandrake control center to set it all up, I have done it several times for me and my friends, first setup eth0 & eth1 as I stated above, then internet connection sharing then firewall, all of them can be setup with the wizards. The wizards will configure DHCP client for eth0 and DHCP server for eth1, it will also setup Shorewall firewall.
Last setup all your hosts in your private network to get ip address automaticly or use DHCP to get an address.