To install addons manually, you first need to find the locations of a few different files.


mainly your steamapps/common/Left\ 4\ Dead\ 2 directory, since all of the other files are within this one.

You can find this buy going to your left 4 dead on steam under your library, then click the cog and then manage > Browse local files

then you need to find gameinfo.txt

steamapps/common/Left\ 4\ Dead\ 2/left4dead2/gameinfo.txt

and your workshop directory

steamapps/common/Left\ 4\ Dead\ 2/left4dead2/addons/workshop

now we are going to make the directory which the mod will be in, in this case mod1.

make this directory
steamapps/common/Left\ 4\ Dead\ 2/mod1

now go to the workshop directory and find the mod you want, you can use the jpgs to find the corresponding mod and copy the vpk with the same number.

then paste the vpk in the newly created directory (mod1)

and rename it to
pak01_dir.vpk

Now we need to edit the gameinfo.txt file

find these lines:


SearchPaths
{
and add on the next one

Game mod1

making sure to use the same syntax and format as all of the other already existing entries
use tabs to match the same spacing.

Now the mod is ready and a restart should ensure it works.

Here is a script I wrote to automate this process.

https://pastebin.com/HcBGnXXk

You will need to edit the user variables at the top probably

download the script/make it (calling it script.sh for example)
open a terminal and run

chmod +x script.sh

then to run

./script.sh