Step 1: Go to YouTube.com, open the video you want to download.
Step 2: In the address bar, add 'ss' between 'www.' and 'youtube'. Your URL will look like this: http://www.ssyoutube.com/watch?v=vDJDvit-dKw.
Step 3: Press Enter and you will be redirected to SaveFrom.net page, a website that allows downloading videos from many famous websites.
Note: By default, the video format selected for you to download is MP4. You can click on the 'More' link to appear more formats like FLV and 3GP (for mobile devices).
Step 4: Select the format and video quality (eg 360p, 480p, 720p) that you want to download. The download process will start soon after.