ClearOS is simply a preconfugred Linux distribution - which comes with a web proxy. There's very little information on the site about what the web proxy is and how the system is configured but the following may apply....
If I were starting from scratch then:
1) block all access to the outside world except for the gateway machine, SMTP server and any machinse which have incoming connections from the internet (and disable routing on the these boxes)
2) Install 'squid' on the gateway machine (see this article for an example of a simple authenticated web access setup)
3) configure the browsers to use the squid server as a proxy (or set up a automatic proxy config file)
There's more information about squid here.