First, decide which other universe you want to crossover with. It could be a game, a movie, or a book. Then, research both universes thoroughly. Know the characters, the storylines, and the rules of each world. For example, if you're crossing with a game like Final Fantasy, understand the magic system and the different races. After that, come up with a plot that makes sense for both sets of characters to be involved in. Maybe it's a threat that only they can stop together.
To start writing a BlazBlue fanfic, you should play the game to really understand the lore and the feel of the world. After that, brainstorm ideas. You could start with a simple concept, say, a day in the life of a minor character but with a twist. Make sure to keep the tone and style consistent with the BlazBlue universe. For example, if the game has a lot of dark and mysterious elements, your fanfic should too.
First, do some research on both BlazBlue and Susanoo. Know their lore, characters, and power systems well. Then, come up with a basic plot idea, like how the two worlds will collide or how a character from one will enter the other. You can start with a simple scenario, such as a BlazBlue character stumbling upon a strange portal that leads to Susanoo's world.
Start with a simple idea, like a meeting between a character from each universe. For example, Yang from RWBY and Ragna from Blazblue. Then, think about how they would interact. Yang might be curious about Ragna's sword and his powers, and Ragna could be intrigued by Yang's strength and her gauntlets. Build the story from there, adding more characters and plot elements as you go.
First, choose the fables you want to cross. Then, think about how the characters from each fable would interact. For example, if you choose 'The Ant and the Grasshopper' and 'The Fox and the Grapes', you might start with the grasshopper meeting the fox while looking for food. Just start writing down these initial ideas.
First, decide which universe you want to cross Monika into. It could be a game, a book, or a movie world. Then, research that world thoroughly. For example, if it's a 'Monika in the Star Wars' crossover, learn about the Force, the different species, and the political systems in the Star Wars universe.
First, decide which world you want to cross Elsa over with. For example, if it's the world of Narnia, think about how Elsa's powers would interact with the magic of Narnia. Then, create a plot. Maybe Elsa is called to Narnia to help the Pevensie children fight the White Witch. Just start with a simple idea and build on it.
First, decide which other show or universe you want to cross over with. For example, if it's 'The Office', think about how the characters from 'Seinfeld' would fit into the Dunder Mifflin office environment. Then, come up with a basic plot idea. Maybe Jerry has to give a comedy workshop to the office employees.
First, decide which universe you want to crossover with. Then, study the lore and characters of both Twewy and the other universe. For example, if it's a crossover with Pokemon, understand how Pokemon battles work and how the Pokeballs function. Then, think of a plot that would bring the characters together in a believable way, like a strange portal opening between the two worlds.
First, decide which other universe you want to cross 'HxH' with. Then, study the lore and characters of both universes thoroughly. For example, if you choose 'My Hero Academia', know the quirks of the 'MHA' characters and how they interact with the Nen abilities in 'HxH'. Next, come up with a basic plot like a common threat that both universes need to face together.
Begin by creating an outline. Outline the main plot points, like how the characters will meet, what their initial reactions will be, and how they'll overcome their differences to fight the common foe. If you're doing a Trollhunters and 'Lord of the Rings' crossover, you might start with the Trollhunters being transported to Middle - earth. Outline how they'll interact with the hobbits, dwarves, and elves. Then, start writing your first draft, don't worry too much about making it perfect at first.