This is pretty easy. You go to a place like youtube.com and grab the embedded link which looks something like this:
<object width="425" height="355"><param name="movie" value="
http://www.youtube.com/v/jKmej7RpPQA&rel=1"></param><param name="wmode" value="transparent"></param><embed src="
http://www.youtube.com/v/jKmej7RpPQA&rel=1" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355"></embed></object>
But when you post it, it just looks like a mess of code. To get it to post you must remove all of the code except for the
orange bit. You don't need both, just one. Then click this

icon from the list. You should get something that looks similar to below.
[flash=200,200][/flash]
Now you need to insert the code from before between the flash code so it looks like this.
[flash=200,200]http://www.youtube.com/v/jKmej7RpPQA&rel=1[/flash]
Now the last step is to make the size of the flash window larger. Default is 200x200 which is small. If you look at the code from youtube you'll notice this bit, <object width="425" height="355">. That's the size of the window. Change 200,200 to 425,350 so it looks like this:
[flash=425,350][/flash]
That's it. Your code should look like this now,
[flash=425,350]http://www.youtube.com/v/jKmej7RpPQA&rel=1[/flash]
http://www.youtube.com/v/jKmej7RpPQA&rel=1