I couldn't find an article like this. So I made up one.

If you right-click an WinRAR file in Nautilus (Ubuntu's file manager) and open it with File Roller, it would tell you that it cannot decompress such kind of file. Ubuntu has no prompt for any steps to fix this.

File Roller, however, is capable to decompress WinRAR files. You just have to install WinRAR for Linux first.

This is actually very simple:

1. Open a browser. (which I'm sure you've done that right now)
2. Go to WinRAR's download page.
3. Download the latest RAR for Linux.
4. Open the folder where you download the package (should be a ".tar.gz" file). Decompress it.
5. You'd have a folder named "rar". Please note down the full path of it. We'll have to use it.
6. Now we do need some console work. Open "Application" > "Accessories" > "Terminal"
7. Remember the full path to "rar" folder? Type these command with your path:
   
   1. cd <full_path_to_rar>
   2. sudo make install
8. ...and its done!!!! Try decompress any RAR by right click and "Decompress here"!