In a Naruto becomes a weapon master fanfiction, he might achieve it through intense training. Maybe he meets a master who teaches him unique weapon skills. For example, he could learn swordsmanship from a hermit in the forest. This new knowledge would add to his combat abilities, making him a weapon master.
Well, Naruto could become a weapon master in the fanfiction by first realizing the importance of weapons. He's always been about his jutsu, but if he starts exploring different weapons, like shurikens with special chakra enhancements, he could gradually become a master. He might also study ancient weapon techniques from old scrolls, and with his determination, master them.
Naruto could become a weapon master in fanfiction by a series of events. He could be sent on a mission where he discovers an old weaponry dojo. There, he meets spirits of past weapon masters who decide to train him. They teach him the art of using various weapons, from simple kunai to elaborate war hammers. He combines his own chakra control and ninja skills with these new weapon techniques, thus becoming a weapon master.