How do I set Masq up?

Choose private network addresses (e.g. 192.168.123.0/24)

Set masq's ethernet to be gateway (e.g. 192.168.123.254)

Setup clients (e.g. 192.168.123.2, 192.168.123.3, 192.168.123.4)

Turn on masquerading in ipchains:

ipchains -A forward -s 192.168.123.0/24 -d ! 192.168.123.0/24 -j MASQ

Turn on IPv4 forwarding through /proc:

echo 1 >/proc/sys/net/ipv4/ip_forward

Always TEST everything! Are you paranoid enough?

Make it permanent (e.g. put it in /etc/rc.d/rc.local)