Set the IP address to the second network interface and connect it to your local network using Switch. Use this IP address in the squid.conf file so that squid listens on this IP address for the connections. This adds a level of security as well.
http_port <Internal_Card_IP>:<port> // the port part can be optional and is defaulted to 3128
Test that internet is working fine on squid system itself. Once this is done, start the squid service.
service squid start
should do it on RHEL clones. If it does not properly start the first time, chances are cache directories are not present.
should create them.
Configure your client browsers to use proxy server and use the above IP address and port configured in squid.conf. And now test internet connectivity.