Well, right now there are 2 ways of doing it:
1st way is the easiest one but not all skins are listed there ---> Steam Workshop
2nd way is downloading them manually, it's a little bit harder, but not too hard

This is the easier way, using the Steam Workshop.
Just go to the workshop , browse what you would like to get, and then subscribe to the item










The weapon skin will then start to download









Then you launch L4D2, and when you press addons, it should be checked, and if you have a few skins for the same game, make sure you enable just 1 of that specific weapon, and disable all others











And then you're good to go, the skin is installed and should be working , just launch the game and enjoy

This is the 2nd way , it's a little bit harder, but not that hard. What you need to do is just go to your browser and type the skin you would like. I personally prefer downloading skins using the gamebanana website[www.gamebanana.com]. Just download the skin you prefer










Then, when you download it, you will get a zip. or rar. folder. In that folder, you will get a source game addon file, that is the file with all of your contect for your skin. You will need to extract the file in my computer > local disk > program files x64 > Steam > steamapps > common > Left 4 Dead 2 > left4dead2 > addons









And then you are good to go, the skins are installed, you can configurate them in the addons when you launch L4D2, enjoy!

Here are some of the links of the skins I like:

AKS-74u[l4d2.gamebanana.com]
M16A2[l4d2.gamebanana.com]
590-Remington defense[l4d2.gamebanana.com]
Barrett M82 50. cal. Sniper Rifle[l4d2.gamebanana.com]

M4A1-S from Insurgency
H&K MP5
ARX-160 From CoD:G and CoD:AW
Honeybadger and Vector CRB from CoD:G



Link for my other guide ---> How to change FOV in L4D2