First make sure you have either WinRAR or 7-Zip.
Download WinRAR here:
http://www.rarlab.com/download.htmDownload 7-Zip here:
http://www.7-zip.org/Then you need to extract the .rar file and move it to your skin folder which should be either "C:\Users\USER\AppData\Roaming\osu!\Skins" or "C:\Program Files (x86)\osu!\Skins"
If it's none of these then open osu! and go into options and find where you change skin. You will see a button bellow that says "Open current skin folder". Click it and it will open the folder in a new window.
Then simply move the extracted skin to your skin folder and restart osu if currently open. Now you should be able to find it in skin options ingame
Edit: Made a little guide instead.