Point your browser at http://192.168.1.1:81 or https://192.168.1.1:445 and you should see IPCop's home page. (Note: v1.4.X will automatically direct you to a secure page). If you can't connect, make sure that your Client PC's Network Settings are set to Automatically Obtain an IP Address, and reboot if necessary, so that the Client obtains an IP Address from IPCop.
Browse to the Services>Proxy page, which deals with the web proxy configuration, and tick the Enabled and Transparent checkboxes, to turn on Squid, the caching web proxy. This will improve performance for clients on the local network, reducing access time as well as bandwidth consumption.
Play with the cache settings to suit your own needs. You can monitor performance by enabling logging on the Services>Proxy page, and checking the performance graphs on the Information>Proxy Graphs page, or studying the logs on the Logs>Web Proxy page.
More information can be found in the Services Section of the Administration Manual.