It was mentioned in two separate lines.
Basically, we already know how to become the Worlds Strongest Swordsman, we saw Zoro attempt that in chapter 51 - by beating the current WSS in a 1v1, not by having better Sword skill than him.
Zoro's interpretation of the title is also simple: "If I'm the WSS, I can't lose to anyone whose a Swordsman" as he told Cabaji.