firstly you need to install youtube-dl. Here’s how to do:

1. Suppose your computer is now running Ubuntu Linux.
2. Open terminal (click Applications > Accessories > Terminal)
3. Type sudo apt-get install youtube-dl.
4. Type in your linux password.
5. Now you’re ready to go. To download your favorite video, simply type youtube-dl -t THE_URL_OF_YOUTUBE_VIDEO. The -t parameter will add video title into the file name.
6. The downloaded file, which is saved in FLV format, then could be
   played using Media Player (in Linux), FLV player (in Windows), or
   another FLV player program.
7. That’s it! Enjoy the videos