Download the zip file and extract it (important). Both these operations can take some time.
Write the extracted image (.img) file to an SD card (minimum size 8GB). Etcher is a great, simple tool for this job.
Insert the SD card into a Raspberry Pi and boot while connected to a LAN port on your broadband router and (optionally) a HDMI monitor.
Note: you must insert the SD card and connect the network cable before powering on the Pi. It may also be a good idea to power on the HDMI monitor first.
The Raspberry Pi will show the server network address on the monitor when booted up. If you have not connected a monitor, don’t worry, you can still find the Raspberry Pi address from your phone using the the Fing Network Tools on Android or iOS.
Connect using the console in your favorite game client with connect <server ip address> eg. connect 10.2.2.185
Note: If you have no client(s) currently installed the quickest “play right now!” option is to visit the nQuake website and grab the appropriate client for your operating system. You won’t need to copy any proprietary files to the game folder and can play QuakeWorld on your network straight away.
You can leave the Raspberry Pi powered on indefinitely if you wish, but check with whoever pays the bill. As a guide, the Pi3B uses only 230 mA when idle (Pi3B+ uses 400mA), which is pretty low compared to other devices (just over $20 per year). It’s about the same as a bedroom clock radio…