Determining when to increase or decrease and how many and when

Trying to figure out the math behind determing when to increase or decrease in a bikini pattern. I am wanting to crochet a bikini and I want to do it in different sizes. Where do I begin? I am not understanding how you get a bikini bottom from a waist measurement and hip measurement. A bikini is not waist high, nor hip low. I would assume it would be considered "high hip." I just don't understand how/where to begin. Hope this makes some sort of sense to someone!! HELP