On your desktop
Download the game
It always plays the newest version, and plays the tutorial and bot games offline.
Your download has started. Open Garden-Wars-windows-setup.exe. If Windows says "Windows protected your PC", click More info, then Run anyway.
Your download has started. Open Garden-Wars-mac.dmg and drag Garden Wars into Applications. The first time you open it, macOS says it can't check the app: open System Settings → Privacy & Security, scroll down and click Open Anyway.
Your download has started. Make Garden-Wars-linux-x86_64.AppImage executable
(chmod +x, or the file's Properties), then open it.
On a phone or a tablet? Play in your browser: nothing to download.
Apple and Microsoft warn the first time you open it, because the app isn't signed with their paid certificates yet.