It's actually pretty easy.
1. Let's say the link to a song is this -> http://www.youtube.com/watch?v=nNC2b6O7a6w
2. You take that link and make it look like this -> http://www.youtube.com/v/nNC2b6O7a6w (so you basically detele the "watch?" in front of the "v", and relace the = with / after the "v"
3. You paste the link you created in step 2 between these two brackets ->
[ flash=425,344 ]x[ /flash ] (you replace the "x" with the link from step 2, and make sure there are no empty spaces inside the "flash" brackets)
4. Final product