go into (or open up) a terminal.
# echo "1" > /proc/sys/net/ipv4/ip_forward
# ipchains -P forward DENY
# ipchains -A forward -i eth0 -j MASQ
this is assuming your internet etherface is eth0 and that you are using ipchains.
you must have your computers behind the gateway set-up to have it as their gateway, and that should be it. easy.
you could also think about running a proxy server, i am running squid on a redhat machine that serves my internet to the rest of my network.
there is also a lengthy thread about just this at www.ms-eradication.org
I cant post the direct link to the thread because this site censors swearing.