Bring up the editor for the page you are wanting to edit.
Click the 'insert media' button 
in the wysiwyg editor toolbar.
Click the web location icon at left hand side of this popup box.
At the top of this popup box, choose from the File Type dropdown box: You Tube Video
At the bottom of this popbox box, paste the url from youtube your video is on (ie. http://www.youtube.com/watch?v=hsvAj6qfmFQ )
Change the width and height to 500 width and 375 height.
Click the insert button.
