iptables - LInux squid proxy

We have RHEL 5.0 squid proxy in our office.

Our LAN is 172.20.12.0 /24 n/w. proxy ip is 172.20.12.4 /24 .

I want to share internet through proxy.

Most importantly:

1. I want to give full access to two IPs 172.20.12.8 and 172.20.12.9 to access the internet


2. All the users should have the proxy address in their browsers to get internet, without the proxy in the browser internet should be blocked.

Can someone help me with what to write in iptables?