To start a 'Future Diary Fanfic', begin by choosing a central theme or idea. This could be something like 'trust' or 'power'. Then, think about which characters you want to focus on. You don't have to include all the main characters from the original. You can also create your own original characters to add more depth to the story. Next, outline the basic plot structure, including the beginning, middle, and end. For example, the beginning could introduce the new rules of the game in your fanfic, the middle could be the competition between the diary holders, and the end could be the resolution of the game.
First, you should re - watch or re - read the original 'Future Diary' to get a better understanding of the characters, the world, and the rules. Then, come up with a unique concept or plot that will make your fanfic stand out. It could be a new character with a special diary, or a different ending to the survival game.