first you hit the youtube button and get this...
youtube=425,350][/youtube except with another bracket on each end
then you take your URL and split it like this
http://www.youtube.com/watch?v= x_wLVCLPx0M&feature=related
and delete everything from Http to the = sign, and paste whats left between the two halves of the youtube "marker" (no spaces tho), and you get
[youtube=425,350]x_wLVCLPx0M&feature=related[/youtube]