Writers blend the two universes by incorporating elements of the Naruto power system into the Until Dawn setting. For example, they might introduce chakra into the Until Dawn world. They also blend the characters' personalities. A Naruto character's determination can be combined with an Until Dawn character's survival instincts.
Writers often blend the two universes by creating a common threat. For example, a demon that has ties to both the Supernatural world and the DC universe's magic or technology. They also use character crossovers, like having Dean Winchester meet Clark Kent. This allows for an exchange of ideas and values between the two different worlds.
Writers blend the two universes by combining the magic systems. In Game of Thrones, we have magic like the wildfire and the warging ability. In Warcraft, there's arcane magic, druidic magic etc. A fanfic might have a character who can use both the warging ability from Game of Thrones and druidic magic from Warcraft, creating a really unique power combination. They also use the geography. Maybe a fanfic has a portal opening between the Seven Kingdoms and Azeroth, and then characters from both sides start exploring the other world. This allows for a lot of interesting interactions and conflicts.
Writers often blend the two universes' aesthetics by incorporating the dark and gritty atmosphere of 'Darker than Black' into the more superhero - centric 'Worm' world. For example, they might describe the cityscapes in 'Worm' with the same sense of foreboding and mystery as in 'Darker than Black'.
One way is by creating crossover characters. For example, a character could have the powers of a Jedi from Star Wars and also be a part of the Avengers team. Another way is to use similar story elements like a fight against a powerful and evil force. Writers can also have the heroes from both universes meet in a neutral location, like a new planet or dimension, and team up to face a common threat.
Some writers use the concept of shared threats to bring the two universes together. In a harem fanfic, this means that both Naruto and Resident Evil characters have to face a common enemy. This enemy could be a powerful being that combines the abilities of the T - virus from Resident Evil and the power of the tailed beasts from Naruto. As they fight together, the relationships in the harem develop, with characters learning from each other's strengths and weaknesses.
Writers can start by researching the lore of both shows thoroughly. They need to understand the rules of the 'Supernatural' world, like how angels and demons operate, and also the nature of the Wesen in 'Grimm'. Then, they can find common elements to build on. For example, both shows deal with creatures that are hidden from the normal human world. Writers can use this shared concept to create a scenario where the characters from both shows meet. Maybe a Wesen is causing trouble that has 'Supernatural' implications, so the 'Supernatural' characters get called in, and the 'Grimm' characters are already on the case.
Fanfiction writers blend the universes by creating new storylines. They might have Naruto end up in RWBY's world due to some magical accident. Once there, he has to deal with Salem's threat. The RWBY characters are initially skeptical of Naruto but soon come to respect his abilities. The different cultures of the universes are also merged. The shinobi way of life from Naruto can be seen rubbing off on the RWBY characters, and the technology and magic of RWBY can be something new and exciting for Naruto. Salem, as a powerful antagonist, adds an extra layer of danger and mystery to the blend of these universes.
Some fanfictions blend the two universes by introducing elements of one into the other. For instance, the Death Note's rules might be applied in the Naruto universe. So a character in Naruto might get hold of a Death Note - like object with similar powers, and then the story explores how that changes the Naruto world's balance of power and relationships. It can also involve characters from Death Note trying to understand the ninja way of life in Naruto, which is full of loyalty, honor, and complex clan politics.
In fanfiction, the two universes blend in multiple ways. The characters from Harry Potter can be put into Creepypasta - like situations. For instance, Harry might find himself in a situation where he has to face a entity that is not from the wizarding world but from a Creepypasta realm and has to use his magic in a new and untested way to survive. Also, the magic in Harry Potter can be warped to fit the spooky nature of Creepypasta. Spells could have unexpected and disturbing side - effects that are more in line with Creepypasta. The creatures in Harry Potter can also be made more menacing by borrowing elements from Creepypasta. For example, a Hippogriff could be made to look more like a nightmarish beast with Creepypasta - inspired features.
Well, first of all, the writers often take the unique elements from each universe. From Naruto, it's the ninjutsu, kekkei genkai, and the concept of the ninja villages. From Highschool of the Dead, it's the zombie apocalypse and the school environment. They then find ways to make these elements work together. Maybe a character with a Sharingan from Naruto can use its powers to detect zombies from afar. Also, they might use the character relationships. A Naruto character could form a team with some Highschool of the Dead survivors to face the challenges together. And they might also play with the different values of the two universes. For example, the loyalty in Naruto and the survival instinct in Highschool of the Dead.