This tutorial explains how to install skins, not just explaining which folder it goes to.
IMPORTANT: Read how to uninstall skins before or after you install a skin.
Note: When you reach your Steam username folder, you might have to create some folders you don't see.

For Weapon Skins

I am going to show you how to install the Gangsta Glock which can be found here

1. First off download the skin
2. Notice the file has a ".rar" extension. Winzip does not support rar files. You must download WinRAR in order to access rar files which can be obtained here.
3. Once WinRAR is installed, open up the file.
4. Now you will see something like Screenshot A
5. Now let's review the folders.
6. materials: this contains the textures for the weapon. Without it, you will have a pink and black looking gun.
• models: skins for models
• weapons: skins for weapons
• v_models: skins for weapons in first person view
• w_models: (this may not appear all the time) Skins for the weapons in world view
7. models: this contains the model for a weapon. Without it, your skin would not work.
• weapons: folder that contains models for weapons
8. scripts: this tells where the sound of each event (reload, draw) goes.
• game_sounds_manifest.txt: Do not overwrite this if you install another skin that has this file in it.
• weapons: sound scripts for weapons
9. sounds: this contains the sounds for the weapon
• weapons: sounds for weapons
• [weapon name]: sounds for this particular weapon

The following procedure (STEPS 6-14) works for any skin that opens up displaying any combination of these folders: materials models scripts sound.
That means you can use this procedure for any skin you download as long as it was submitted correctly.

For Player Skins

1. Download a T/CT skin
2. Open with WinRAR
3. You should see some vtf files
4. Go to C:\Program Files\Valve\Steam\SteamApps\Counter-Strike Source \cstrike\materials\models\player\
   The folder above may be different for you.
5. Create these folders: ct_gign ct_gsg9 ct_sas ct_urban t_arctic t_guerilla t_leet & t_phoenix
6. If the files in the rar file start with cturban, drag and drop all the files into the folder "cturban". If the files start with t_phoenix, drag and drop all the files into the folder t_phoenix. The same concept applies to the other Terrorist & Counter-Terrorist skins.

Scopes

1. If you end up with a scope_lens.vtf when you download a scope, extract it to ...\counter-strike source\cstrike\materials\overlays\
2. If you just have a materials folder, drag and drop the materials folder onto the background of the cstrike folder. (\counter-strike source\cstrike\)
3. The material file is located in cstrike\materials\overlays\scope_lens.vtf

Shells

1. If you end up with a rar file that only shows the title of the release when you open it, double click that folder and you should select the materials (and models if applicable) folders and drag them to the background of the cstrike folder. Yes, you do overwrite all.
2. The shells are stored in cstrike\materials\models\shells\. Let's review the folders within shells. a. 12gauge - for shotguns b. 338mag - for awp c. 556 - for rifles mostly (some include m4a1, m249, galil) d. 57 - for fiveseven and p90 e. 762nato - for rifle ak47 (maybe others) f. 9mm - for glock18, tmp, mp5navy (maybe others)
3. There is also a special folder for the deagle shell. It is stored in materials\models\v_models\pist_deagle\ and is called bullet.vtf.

1. The hands/gloves texture is cstrike\materials\models\weapons\v_models\hands\v_hands.vtf

Flashlights

1. The flashlight file is located in ...\cstrike\materials\effects\ and is called flashlight001.vtf.

Muzzle Flash

1. The muzzle flash file is located in ...\cstrike\materials\effects\ and is called muzzleflashx.vtf.

Technical Support
Q : My skin turned out to be pink and black checkered in the game and I know I installed it right! WTF??????
A : In most cases someone didn't submit the file right. I know in some skins (like the black m134 Vulcan mini gun ) you get pink and black textures. Fortunately, there is way to fix it!
1) Go to counter-strike source\cstrike\materials
2) Right click the folder weapons and click cut. (Control+X)
3) Now go to ...\counter-strike source\cstrike\materials\models\
4) Right click the background of this folder and click paste (Control+V) and select yes to all.
5) Now your skin is fixed
In other cases the folder that you have to move will be somewhere else. The easiest way to find out is to open the zip/rar file for the skin, and then explore into the materials folder to see what folders were placed wrong.

Q : All I see is a cstrike folder when I open up the skin file!
A : Then just drag and drop cstrike to ...\username\counter-strike source\

Q : I did everything you said and it still doesn't work! If you're just going to message that you better include what skin you were trying to install. I will determine if it is your problem or the compiler's problem.

Q : What if it is the compiler's problem?
A : I'll tell you if it's easy enough for you to fix in the same post.

Q : What if it is not easy enough to fix?
A : PM the submitter to fix it.

Q : What if the submitter is a guest?
A : Then just do the work yourself; go watch SilentAssassin12's hacking/compiling tutorials.

Please, don't keep posting, "I did everything and it still doesn't work!!" without even giving a link to the CS:S skin you were trying to install.

Tutorial by xz64
Tutorial fixed by BigDog