First you need to find the youtube VIDEO ID of the video you would like to embed - see below

Then you need to copy that to your clipboard (Highlight the ID & CTRL + C)

Now, start your new thread for the video and find the "Youtube Video Embed" button on the toolbar

Then once you have pressed the button you'll see the Youtube Tags appear, like so :

Next step is to paste the Youtube Video ID inbetween the tag (between the ][ in the middle)

Then post the thread, and you're all done... Video is embedded...