1. Enter your RPC details of choince (you must pick them for use in miners later) in your bitcoin.conf
(it can be in ~/.bitcoin on Linux, ~/Library/Application Support/Bitcoin on Mac, not sure where on Windows) on your server
2. Start a bitcoin server
on one computer:
The former starts in "background" and doesn't display any GUI, the latter displays like normal client, but allow external connections.
3. Start miners
on your clients, for example using poclbm:
poclbm -rpcuser=YOUR_USER -rpcpassword=YOUR_PASSWORD -rpchost=YOUR_SERVER_IP
And you're done!