How to download Half-Life 2 mods that don´t appear in the workshop?

First, you need the main game for sure. Some mods require the episodes too, so just install them in case. You can find Half-Life 2 with both episodes here:
https://store.steampowered.com/app/220/

Read this carefully!

Before you start, make sure that you have "Source SDK Base 2013 Singleplayer" installed, you can find it in your steam-library by typing in "source" .
After its succesfully installed, right-click it, go to properties>beta and select "upcoming".

If you want to run older mods, install "Source SDK Base 2006" and "Source SDK Base 2007" too.

You will also need a program like win-rar[www.win-rar.com]or 7-zip[www.7-zip.org] to un-zip the mod-folders later.

Simply download them on a website of your choice, i would recommend trying Moddb or Gamebanana[gamebanana.com]but its up to you.

After you have downloaded a mod, you should have a file of it like this. Example:

Now just extract it by righclicking>extract all and you´re ready for the next step!

Rightclick Half-life 2 in your steam-library, then "manage">"browse local files". Now double-click the arrow, that takes you back one folder, 2 times. There will be a folder called "sourcemods" .
Take the main file from the mod of your choice, where the data is stored (not the "read me"-file or sth like that) and drag it into the "sourcemods" folder.
After restarting Steam, the mod should either appear in your steam library or is already in the game. For example:

If you can´t find the "sourcemods"-folder, then open the explorer, scroll down and click on "this pc>Volume where HL2 is installed on> programs (x86)>Steam>Steamapps> Sourcemods

If done correctly, the mods should work (remember to restart steam!)

If the mods dont work, remember: Active workshop-mods are always above external mods, try turning them off if something doesn´t work!