Installing a skin is very easy but it depends on file extension if it's iwi file read paragraph number one, else if it's iwd file read paragraph 2

Paragraph 1

1. Put your iwi file into CoD4/mods/ModWarfare/images
2. Delete CoD4/mods/ModWarfare/z_modwarfare.iwd
3. Click makeMod.bat and wait for compression.
4. Open z_modernwarfare.iwd file with winrar and if there is a folder z_modwarfare/maps delete /maps
5. Start the game, launch mod type in console /devmap mp_yourmap

If you don't understand there is other possibility to install IWI skins:

Open up your_mod.iwd (with winRAR) open folder called images and copy your IWI skin to images folder without compression!!! (Store option in winRAR)

Paragraph 2

1. Copy your skin.iwd to CoD4/mods/your_mod/skin.iwd
2. Start game, launch your mod
3. Type in console /devmap mp_yourmap
4. Have fun :)