1. Go to your osu! folder
2. Go to "Skins" folder
3. Find the skin that you download (the folder one)
4. There's the inside of the skin
EDIT: for .rar, well here's how:
1. Open the .rar file that has skin inside
2. Click the folder ONLY and extract it to your osu! folder, Skins folder
3. Same instruction above that I have given you before I make this edit
If the .rar has no folder, but only the elements, here's how:
1. Open the .rar file
2. Select all elements (except the "..." one)
3. Before you extract it, go to the location at Skins folder in your osu!. Then, make a new folder and make any name that you want to insert
4. Click the folder that you made and extract it
5. Follow the steps above (before EDIT)