When you have installed and enabled the plugin and you have setup the custom field, you can start using it.
Go to the article you like to add the Youtube video.
You see a new tab. In this example it's called "Video" but it's the name you choose for the the Custom Fields Group.
You also see the description if you entered one in the Custom Fields Group.
Youtube video: Here you enter the ID of the Youtube video.
You can find the ID of the Youtube video in the Youtube URL.