Well, first, think about what kind of story you want to tell. Is it a fictional adventure, a real-life event, or something else? Then, break it down into a clear beginning, middle, and end. Also, pay attention to your language and try to make it interesting and easy to follow.
Well, in 'kimetsu no yaiba the first novel', it probably focuses on the early days of the main characters. It could be about Tanjiro's journey as he first discovers the existence of demons and decides to become a demon slayer. It might also include his encounters with other important characters like Nezuko, and how their relationship develops in this different narrative format.