Well, initially, you need to be really familiar with 'ahs coven'. Know the strengths and weaknesses of each character, their relationships, and their goals. Then, when choosing what to crossover with, think about things that would create an interesting contrast or synergy. For example, crossing with a dystopian world. Start writing by just jotting down some key moments or interactions you can imagine. Maybe start with a conversation between a 'ahs coven' character and a character from the other world. From there, you can build out the full story, adding details about the settings, the reactions of other characters, and how the crossover affects the overall plot.
First, decide which other universe or story you want to crossover with. Then, research both 'ahs coven' and the other element thoroughly. This will help you understand the characters, settings, and rules of both. After that, come up with a basic plot idea that involves the crossover. For example, if you're crossing with 'Buffy the Vampire Slayer', think about how the witches and the slayer could interact. Start with a strong opening scene that sets the stage for the crossover, like the coven witches suddenly appearing in Sunnydale.
To start writing 'ahs coven crossover fanfiction', begin by brainstorming the main conflict that will drive the story. Consider the personalities of the 'ahs coven' characters and how they would react to the new situation in the crossover. You could also create new characters that are a blend of both universes. For instance, if crossing with 'Charmed', you might create a character who has the powers of both a 'ahs coven' witch and a Charmed one. Don't be afraid to play around with different ideas until you find one that really clicks.
First, research both the character of Kit and the world or characters you want to cross him over with. Know their traits, backstories, and what makes them unique. For instance, if you're crossing Kit with the characters from 'Hotel', understand the dark and often seedy nature of the Hotel and how Kit would react to it.
First, know the 'American Horror Story' universe well. Watch all the seasons you want to base your OC in. Then, create your OC. Decide on their appearance, personality, and backstory. For example, if it's for 'Hotel' season, your OC could be a vampire hunter with a tragic past that led them to hunt vampires. Next, think about the plot. How will your OC interact with the existing characters and the setting? You can start with a simple idea like your OC checking into the hotel for a specific reason and then things start to go wrong.
A cool 'ahs coven crossover fanfiction' idea could be a crossover with a fairytale world. The coven witches could find themselves in a land of fairytales. They might meet characters like Snow White or Cinderella. The witches' powers would be very different from the magic in the fairytale world, and this contrast could lead to some exciting adventures. Maybe they have to help a fairytale character break a curse using their unique witchcraft.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfictions, and you can easily search for 'Harry Potter AHS Coven' there.
You can start by looking on fanfiction - dedicated websites such as Archive of Our Own. They have a tagging system, and you can search for 'ahs coven cordelia' and exclude any tags related to sex.
Begin by creating your main characters from both universes. If it's a Splatoon and Sonic crossover, think about how the inklings would interact with Sonic and his friends. You can start with a simple situation like a race where the inklings' ink abilities give them an advantage in some parts of the track. Then build on that, adding more complex story elements as you go.
First, decide which two (or more) universes or characters you want to cross over with Venom. Then, research those characters or universes thoroughly. For example, if it's a Venom and Star Wars crossover, learn about the Star Wars lore. Next, come up with a basic plot, like Venom somehow ending up in the Star Wars galaxy and getting involved in the conflict between the Rebels and the Empire. After that, start writing the opening scene, introducing the situation or how Venom arrives in the new setting.
First, decide on the two (or more) universes you want to cross over. If it's centipede and a magical realm, start by researching centipede characteristics. Then, think about how the centipede would fit into the magical world. Maybe it could be a familiar for a young wizard. Just start writing down ideas about how the centipede's physical features and behavior would interact with the new setting.
Begin by brainstorming the characters and settings you want to combine with Luigi. Let's say it's a crossover with the Final Fantasy series. Think about how Luigi would adapt to a world full of magic and complex characters. You could start with Luigi being magically transported to a Final Fantasy realm. Describe his initial confusion and then start introducing the main conflict. Maybe a powerful sorcerer in the Final Fantasy world is trying to take over the Mushroom Kingdom too, and Luigi has to find allies in this new world to stop him. Develop your characters' personalities and relationships as you write, and don't be afraid to let the story take unexpected turns.
Begin by brainstorming how the different elements of the two crossovers will interact. If it's mcsm and the Legend of Zelda, consider how the tools and abilities in mcsm compare to those in Zelda. You could also look at the personalities of the characters. For instance, if Link is quiet and reserved, how would he interact with the more talkative mcsm characters? Once you have these ideas, start writing the first chapter, introducing the setting and the main characters from both universes.