The Best Way On Earth To You Start To Choreograph Dressage Musical Freestyle?

I have an interest in dressage, and my horse is trained really well in it, and so far our lessons have been going really well. I would love to compete in musical freestyle. I even have a song I would enjoy using (its from the Harry Potter and the Half-Blood Prince soundtrack, "Journey to the Cave" lol)

But I’m totally new in this, so I don’t know where to start! First of all, does 4-H usually have musical freestyle? I mostly show 4-H.

Second, how long is a routine/test? (and what do I call it? Routine? Test?)

How do I start figuring out what maneuvers to do and in what order?

Any help or advice would be great! And, I am taking lessons, btw.

Just for fun, here’s the song I wish to use: http://www.youtube.com/watch?v=xwUIgA4LGOo
(please let me know if this isn’t the kind of music they’re looking for or something, too!)

Thanks, guys!
There is definitely 4-H dressage in my state, there’s just separate shows for it.

The Best Way On Earth To You Start To Choreograph Dressage Musical Freestyle?