You will automatically have a playlist for your account called ‘Watch Later’. This Playlist can be used immediately allowing you to add videos you wish to watch, but may not have time to there and then.  To add videos to your Watch Later playlist simply click on the ‘Watch Later’ icon where ever you see the video listed: 

Or click the ‘Watch Later’ option below the video player:

The ‘Watch Now’ playlist works dynamically, meaning when you’ve viewed a video from the list it is automatically removed.