You should also do some research on both universes. If you're crossing Xander with the 'Doctor Who' universe, know about the rules of time travel in Doctor Who and how Xander's normal life in his own world would be affected by it. Then, start creating a plot that involves Xander as the central character. Maybe he gets accidentally transported to the Doctor's world and has to find a way back while also helping the Doctor save a planet or something. Build up the story around his experiences, his reactions to the new world, and how he uses his own abilities to get through various challenges.
Begin by brainstorming how Xander's personality would fit into the new setting. If it's a crossover with 'The Lord of the Rings', Xander's courage would be a great asset, but his modern - day thinking might clash with the more traditional values of Middle - earth. Develop a conflict or goal for him in this new world. It could be that he has to help Frodo get to Mordor in a different way than in the original story, using his own skills.
First, you need to have a good understanding of the Buffy universe. Know the characters, their personalities, and the rules of the Buffy - verse, like how vampires are staked. Then, choose the other universe you want to cross it with. Decide on the main plot points that will bring these two universes together. For example, if it's a crossover with 'The X - Files', you could have a plot where a demon is causing strange occurrences that Mulder and Scully are investigating, and Buffy gets involved.
First, think about Mina's character traits. She's bubbly, energetic, and has a unique Quirk. You could start with a scene that showcases these traits, like her waking up excited for a new day at U.A. High School. Then, come up with a simple plot. It could be as basic as her trying to make a new friend or improve her skills in a particular area.
There might be a Xander - centered crossover fanfiction where he enters the 'Harry Potter' world. He could bring his knowledge of fighting the supernatural from his own world to Hogwarts. Maybe he'd help the students there deal with some dark magic threats that the teachers are having a hard time handling. This type of crossover can create really interesting storylines as Xander's different perspective and skills add a new dynamic to the wizarding world.
First, you need to create an outline. Decide who Xander is in your story, his personality traits, and what kind of harem he will have. For example, if it's a fantasy setting, his harem could be made up of different magical beings.
You should also focus on character development. If you're crossing with the Fullmetal Alchemist world, consider how Chrollo would interact with the Elric brothers. He might see their search for the Philosopher's Stone as something he can use for his own purposes. Start with a small scene that sets the tone, like Chrollo arriving in Amestris and observing the alchemic practices there, and then build the story from that initial encounter.
First, decide which universe you want to crossover with Tracer. Then, research that universe thoroughly. For example, if it's a Star Wars crossover, know the rules of the Force, the different species, and the technology. After that, create a plot that involves Tracer being thrust into this new universe. Maybe she's chasing a villain through a wormhole that leads to the Star Wars galaxy. Start with an engaging opening scene, like Tracer suddenly appearing in the middle of a lightsaber duel.
First, decide which universes or characters you want to crossover with Gogeta. Then, think about the plot. It could start with a disruption in the Dragon Ball universe that leads Gogeta to another world. For example, if it's a crossover with the One Piece universe, it could be a mysterious energy that pulls Gogeta to the Grand Line.
First, decide which universe you want to cross Ichigo over with. Then, research the rules and characteristics of that universe. For example, if it's a crossover with the Star Wars universe, learn about the Force and the different species. Next, think about how Ichigo would fit into that world. What would his role be? After that, start with an interesting hook in your story, like Ichigo suddenly appearing in a strange new place.
First, decide which universe Gowther will cross over into. Then, think about how Gowther's character traits and powers will interact with the new world. For example, if it's a crossover with 'Naruto', Gowther's power to manipulate memories might be a big deal in a world where knowledge and hidden pasts are important. Just start with a simple scene, like Gowther arriving in the new world.