Begin with a strong hook. It could be a situation that immediately throws the characters from both universes together in an unexpected way. For example, if it's a crossover with 'My Hero Academia', start with Ciel and Sebastian suddenly appearing in the middle of a villain attack at UA High School. This will draw the readers in and make them want to read more.
Start with creating your characters. Even if you're using the existing characters from Kuroshitsuji, you might want to add some new traits or backstories to them. For example, if you're writing about Grell Sutcliff, you could give him a different motivation for his actions. After that, plan out your plot. Think about the beginning, middle, and end of your story. Make sure there are some interesting twists and turns to keep the readers engaged.
First, you need to have a good understanding of the Kuroshitsuji universe. Know the characters, their personalities, and the rules of the world like the existence of demons and reapers. Then, come up with an interesting concept. It could be a new adventure for Ciel and Sebastian, or a what - if scenario like what if Ciel never made the contract with Sebastian. After that, create an outline of your story, including the beginning, middle, and end.
Another idea is a crossover with 'Fullmetal Alchemist'. Ciel could be interested in the alchemy in that world and perhaps try to use it for his own purposes. Sebastian would be there to either help or hinder depending on his own motives.
To start writing a chara crossover fanfiction, you should start with brainstorming ideas for how the characters will interact. Will they be friends or foes at first? Also, think about how the different rules and worlds of the original characters will blend or clash. For example, if you're crossing a magical character with a non - magical one, how will that work? Sketch out a rough outline of the story, including the beginning, middle, and end. You can start with a simple premise like 'What if these two characters met?' and then build on it. Consider the character arcs for each of the crossed - over characters. How will they change during the story?
Begin by brainstorming ideas. Think about what would happen if the 'legion' met a different group of characters. You can draw inspiration from other crossover fanfictions you've read or movies/TV shows you've seen. After that, develop your characters. If you're bringing in characters from different universes, give them new dynamics. They might be initially suspicious of each other but then learn to work together. And don't forget to set the scene. Describe the new world that is created by this crossover, whether it's a mix of the 'legion' world and another or a completely new place where they all end up.
To start writing 'Battletech Crossover Fanfiction', begin by choosing a crossover that has potential for great stories. Say you pick 'Battletech' and 'Full Metal Panic'. Research both universes thoroughly. Next, create a strong plot. Maybe the mechs are being used in a war on a planet similar to the one in 'Full Metal Panic'. Develop your characters well, like how a Battletech pilot would react to the situations in the other universe.
Begin by exploring the personalities of Robin and the character he is crossing over with. If it's, say, Robin and Supergirl, understand how Robin's detective skills and combat training would interact with Supergirl's superpowers. You can start with a small encounter between them, like they meet by accident and then gradually build up to a bigger adventure. Also, don't forget to add some conflict, whether it's internal (Robin struggling with his own insecurities) or external (facing a powerful enemy).
Begin by creating a character profile for Android 21 in this new crossover setting. Consider her goals and how they might change. Also, establish the rules of the new world she's in. If it's a world of magic, how will her powers interact with magic? You can start with a simple scene, like her arrival in the new world, and build from there.
Begin by brainstorming interesting scenarios. If you're doing an 'It' crossover with a sci - fi story, think about how the otherworldly elements of 'It' like the sewers and Pennywise's powers would work in a sci - fi setting. Decide on your main characters from both universes. Then, create a rough plot outline. You could start with a character from one world stumbling into the other. It's all about being creative and having fun with it.
First, decide which two universes you want to cross over. For example, if it's Little Nightmares and Alice: Madness Returns, study the lore and characters of both. Then, create a scenario that brings the characters together, like a strange portal opening up.