Well, to start writing an Undertale fan fiction, you need to have a passion for the source material. Begin by jotting down some ideas about the story you want to tell. Maybe you want to focus on a particular character's backstory that wasn't fully explored in the game, like Muffet. Then, think about the plot structure. Will it be a linear story or more of a episodic one? And don't forget to add your own unique spin to it. You can change the relationships between characters slightly or introduce new elements to the Underground. For example, you could create a new type of magic that only certain characters can use. Once you have all these elements in place, start writing and let your creativity flow.
Begin by choosing a theme for your fan fiction. It could be friendship, like how the friendship between Frisk and the monsters could be developed further. Next, decide on the point of view. Will it be from Frisk's perspective, or perhaps a third - person omniscient view? Research the existing fan fictions out there to see what has been done before and what you can do differently. Outline your story, including the beginning, middle, and end. In the beginning, introduce your main characters and the situation. In the middle, build up the conflict, like a new threat to the Underground. And in the end, resolve the conflict in a satisfying way. Also, make sure to use the unique style of Undertale in your writing, with its humor, heart, and charm.