A Raley is an edging/leverage trick. Therefore, yuo should be good at both. You definetly need to be comfortable with your HS Wake jumps, and shoud feel comfortable taking them out into the flats. You should also have a nice progressive edge. Once you have all of that, then you should have enough to do a raley.
I would advise to look for instruction somewhere (either through DVD, the internet, magazine, teacher, etc) before trying the trick. Just don't go into it cold, not understanding how the trick works. Also, if you have access to a trampoline, doing it on there first will help you greatly.