Download

classicQ needs the original game data: pak0.pak and pak1.pak.
Both come with every copy of Quake - grab one on Steam or GOG if you don't own it yet.

Install the game and find where Steam or GOG put it on your disk.
Inside sits an id1/ folder with both paks - copy them somewhere handy, you'll need them in a minute.

Optional: Verify pak files

Want to double-check you've got the correct files? Compare their MD5 hashes against the table below. If they match, you're good to go.

You can easily calculate MD5 checksums with PowerShell on Windows:
Get-FileHash pak0.pak, pak1.pak -Algorithm MD5

With Terminal on Linux:
md5sum pak0.pak pak1.pak

Or with Terminal on macOS:
md5 pak0.pak pak1.pak

Put everything together

You're almost finished! Now grab the latest classicQ release from GitHub:

Unzip it anywhere you like, then drop pak0.pak and pak1.pak into its id1/ folder.
Rename them to lowercase if they aren't already.

macOS: Gatekeeper might claim the downloaded app is "damaged". It isn't - the app is just not notarized with Apple. Clear the quarantine flag and it runs fine:
xattr -cr classicq-macos-arm64.app

Congrats! You have finished installing classicQ!
To start the game, go to the unzipped classicQ directory and run it. Happy fragging!

BTW, if you're curious about the code, it's all on GitHub. You can even compile classicQ yourself.