It's not that complicated. Go into the full editor first and follow this:
1) Click the "Flash" tab at the top
[flash=][/flash]
2) Copy and paste your YouTube link
[flash=]https://www.youtube.com/watch?v=pco91kroVgQ[/flash]
3) Enter the size of the video player (highlighted in blue)
[flash=500,302]https://www.youtube.com/watch?v=pco91kroVgQ[/flash]
4) Delete the text highlighted in red...
[flash=500,302]https://www.youtube.com/watch?v=pco91kroVgQ[/flash]
5) Now put a forward slash after the "v" highlighted in green
[flash=500,302]https://www.youtube.com/vpco91kroVgQ[/flash]
And voila.