There's a little button that looks like a film strip - it's right next to the "insert/edit image" icon - all you do is click that and a little dialogue box will come up. Put the entire youtube link in and then hit: "embed". That should just about do it...
Or, if you prefer using tags, you could quote a post that contains a video, see how the tags are set up and put in the tags the old fashioned way. It's really all about preference.