Hi.
Title says it all. Don't forget that you will be the only one who can see the skins.
It's a simple 10-steps guide.
1. Go here:
C:\Program Files\League of Legends\Game
2. Find "HeroPak_client.zip".
3. Copy it and paste it somewhere, not in the lol folder.
4. Open it and go to characters.
5. You will find many folders with champions' names.
6. The skins are .dds files. For example I open Tristana's folder. You will see a file named "Tristana.dds".
That's the original. The custom one is "Tristana_earnestelf.dds". Rename it to "Tristana.dds".
7. Go to:
C:\Program Files\League of Legends\Game\DATA
8. Make a new folder and name it "Tristana".
9. Add there "Tristana_earnestelf.dds".
10. Run the game.