Don't! Let the cruise line select it for you! All cabins within the same class category are pretty much the same. You sign up for the best available cabin within your class - say a balcony cabin. You are guaranteed to get a cabin with a balcony somewhere on the ship, but you pay rock bottom price. Chances are you will be given an upgrade - at least a better location. I am 8 days out from my next cruise, and instead of the cheap balcony cabin we are paying for, we have been given a big mini-suite with full bath and large balcony! Woo-hoo! Just found out today!
In the 17 cruises we have taken, we have never once been given the cabin that we actually were paying for! We have never had a bad cabin, either.