Dean. Here is a picture tutorial!! 
Step one.. Press the Xmas tree icon and a box will come up.
Click on 'External Site'
Step Two... Make sure you have copied the video URL you want to show, and paste it into the box that says URL.
Where it says 'Type' click the arrow on the end, and select Youtube.
Then Click insert!!
It will then go into your post, but look like this until you press Sumbit reply...
And that's it!
