you do it using bbcode.
[youtube ] [/youtube ] (minus spaces, of course)
between the tags, you put the video's id. Look at the URL of the video you want to embed, such as:http://www.youtube.com/watch?v=M82CUd6isyY
the letters and numbers after the "...v=" is what you want. In this case, M82CUd6isyY