How to run oclhashcat on iTunes backup password?

I have an iPhone backup with unknown password and I badly need to look inside it 🙂 So I guess the best way in this case is to run brute force attack on it…
Yes, I have access to Google ( 🙂 ) and read some articles about it, but I have no idea how to do it in right way. I learned it uses pbkdf2 algo for crypt.
Is there any one who can explaine to completely noob how to write the command string to run oclhashcat? What kind of data is needed for that and how I can get this initial data from given backup?
Thanks for you time! 🙂