Go to the YouTube video page that you want to embed, then copy the link in the "URL" field (sometimes it's different from the one in your address bar). It's right above the "Embed" code.
Now, take the link:
...and delete everything up to and including the equal (=) sign, leaving you with this:
T4zEE3FC7kQ
Click the YouTube button, or type out the opening and closing YouTube tags, placing the string of numbers/letters inside:
[youtube=425,350]T4zEE3FC7kQ[/youtube]
[youtube=425,350]T4zEE3FC7kQ[/youtube]
All set.