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:
rpcuser=YOUR_USERNAME
rpcpassword=YOUR_PASSWORD
2.
Start a bitcoin server on one computer:
bitcoin -daemon
OR
bitcoin -server
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!