To start a twewy fanfic, begin by choosing a perspective. Are you going to write from a Player's point of view, like Neku, or a Reaper's? After that, think about the setting. You can either stick to the familiar areas in the game or create new ones. For example, you could describe a hidden part of the Underground. Then, start writing down ideas for the plot. Maybe it's a mystery that the characters need to solve, or a conflict between different groups in the twewy universe. Don't worry about making it perfect at first; just get your ideas on paper.
Well, if you want to start writing a twewy fanfic, immerse yourself in the twewy fandom. Read other fanfics to get a sense of what's been done and what you can do differently. Once you're ready, think about the theme you want to explore. It could be related to the game's themes of death and rebirth, or something more unique like a crossover with another game world. Then, start building your characters. You can add new traits to the existing ones or create entirely new characters that fit into the twewy universe. After that, start writing the first chapter, focusing on introducing the setting and the main characters.