Go to the article you want to embed the video.
Click Edit Article (link above or below the article).
Gallery Panel section
Click on Add Gallery button
Add a video from YouTube, Vimeo or Wistia: paste the YouTube video link
Click on the Add Video button
Main Article Image section
Select Video/Animated gif
Select Show main article image in article list - if you want this thumbnail to be displayed on the main article list page.
Follow all the above steps of adding the video to an article
Action section (in gallery panel)
Copy the text from the textbox
Paste it in the body of the article
Follow all above steps
Custom fields section
Video Size dropdown: select 75%
Update article.
Review.