Search in google. Google is your friend
How to install skin:
1. Find the download link
2. Download it until it finishes
.Osk File:
Just click the .osk file
.rar file:
1. Click the .rar file
2. Extract it to your osu! file, skins
3. If it's not available in your skin selection (but it is working), restart your osu! (Same thing with .osk file)