What you need:
-League of legends
-Brain.exe
Go to your lol files ( for me it's C:\Program Files (x86)\League of Legends ) and click on Game and following on HeroPak_client.zip.
Extract the file "Character" in your desktop and open it. Go to C:\Program Files (x86)\League of Legends\Game\DATA\Characters ( If you don't have the file "Character" in your lol file, create it ) and go to the character file you want the skin and add the skin of the file character in your desktop and rename it to the name of the character.
Example: I want the skin Italy of Kassadin.
I go to the file Voidwalker in my desktop ( C:\Users\Pol\Desktop\Characters\Voidwalker )
I take the file Kassadin_italy.dds and I add it in C:\Program Files (x86)\League of Legends\Game\DATA\Characters\Voidwalker and I rename it to Kassadin.dds
Launch the game and have fun!
Sorry for my bad english, i'm french.
I add a video later, wait for please, it will explain better.