use the tags [.yt] and [/yt] then just put the youtube video ID in (get that from the web address at the top) so for example it might look like [.yt]rf567h4s[/yt] (but without the full stops!)
watch this one till the end. backstory - people think they are in a sauna... but little do they know. the end ones are amazing! [yt]7BtNN6M97q8[/yt]