Amxmodx is a plugin for Metamod.
Amxmodx has it's own scripting language and has it's own installing process.
Unlike Metamod plugin, Amxmodx plugins requires a little bit more effort to install.

First, find yourself a plugin from: 
https://www.amxmodx.org/compiler.php

You can find several plugins for several different gamemodes.
Some of the plugins works only the a single gamemode (Etc Counter Strike 1.6, Team Fortress Classic)
While some of the works globally.

(If you press to search without typing anything, it will show ever existing plugin.

Simply download any plugin that you want (warning, some plugins might not work or can crash your game)

Now take the downloaded plugin and copy-paste it into here (Half-Life 1 example)

Half-Life/valve/addons/amxmodx/plugins
Once your put your plugin there, we need to activate it.

Go to

Half-Life/valve/addons/amxmodx/configs
Open the file "plugins" with notapad or any other text editor.
As long as you add the exact name of the plugin in this file, it will work.
It doesn't matter where you add.

For example, I added a plugin named "cheat".
Allowing you to use godmode and noclip in Multiplayer servers, which is not available normally.

I simply added the exact name of the plugin like this:

cheat.amxx

After that, launch your game and test your plugin.
If the game doesn't crash than you have succesfully installed your plugin.
