You're probably here because you; don't trust my script (don't blame you), Have a non-default steam install location, or the scripts didn't work.
The installation is standard and mostly the same as most sourcemods if you have experience.
Start by downloading lowerzone, then extract the zip.
Copy the lowezone folder to steam\steamapps\sourcemods, by default steam is found In (This PC) C:\Program Files (x86) but it can be wherever you installed it, make sure your sourcemods shows lowerzone and not bin, materials, etc.
When you are done copying the lowerzone folder to sourcemods restart steam by quitting it and then opening it again (just closing the window isn't enough you have to either kill steam with task manager or quit steam from the applet on the taskbar).
After steam loads make your way over to the library and search for Source SDK 2013 Multiplayer and hit download if you haven't already. You should also download and install Team Fortress 2 (technically optional) as lowerzone uses some of its assets.
Hit play on lowerzone and have fun, click find servers to join someone elses game or create your own server with the Create Server button (Be sure to check steam networking as it puts your game on the Find Servers section).
It mostly the same steps just remember that steam installs in .steam on most distros if you aren't using flatpak
On Steam Deck you might have to play around with binding controller buttons to keyboard and mouse with Steam Input