By using the characters' unique abilities. For example, Naruto's ability to quickly learn new skills could be used when he's in Middle - earth. He could learn some elvish magic or the ways of the Rohirrim. And the hobbits' stealth could be something that Naruto admires and tries to incorporate into his ninja tactics.
Themes are important. The theme of friendship in Naruto is similar to that in Lord of the Rings. So in fanfiction, the characters from both universes can bond over their shared values. Also, the power of determination in Naruto can be paralleled with the hobbits' determination to destroy the One Ring. This way, the two universes can be combined well.
Often through character crossovers. They put characters from both series in the same story. Like Harry and his friends could end up in Middle - earth, or the hobbits could find themselves at Hogwarts.
Well, one way could be through a crossover spell or some sort of interdimensional rift. Maybe a character from Twilight somehow ends up in the Naruto world and has to adapt to the ninja way of life.
Often, it combines them through crossovers. For example, a portal could open between the Naruto universe and Shinji's world, allowing characters to travel between them.
The 'Harry Potter Fire Lord' fanfiction can combine the two universes by creating a portal or a magical accident that transports characters from one world to the other. For instance, a spell gone wrong in Hogwarts could send a character to the world of the Fire Lord.
The combination also happens in terms of power systems. Ninjutsu from Naruto and bending from Avatar can be merged in interesting ways. For example, a character could try to use chakra to enhance their bending abilities or use bending techniques to improve their ninjutsu. This creates a unique blend of the two universes in the fanfiction. In the harem aspect, characters with different powers can attract each other, leading to more complex relationships. And as for the 'lemon' part, which is often about more mature or romantic content, it can add a different layer of relationship dynamics between the characters from the two universes.
They blend by combining characters. For instance, having Frodo in Westeros or Tyrion in Middle - earth. Also, the settings can be merged. The dark forests of Middle - earth can be made similar to the haunted woods in 'Game of Thrones'.
It can combine the two by having the characters cross over. For instance, Harry could be invited to MI6 to share his knowledge of dark magic as they face some sort of dark magical threat in the Muggle world.
One way is through power systems. The biotics in Mass Effect and the chakra in Naruto can be related. Maybe a character in a fanfiction discovers that biotics are a form of chakra or vice versa. Also, the different races in Mass Effect and the clans in Naruto can interact in terms of politics and alliances. For instance, the Uchiha clan could form an alliance with the Salarians based on their shared intelligence and strategic thinking. And the stories can also use the different settings, like having a battle take place on a Mass Effect planet but with Naruto - style combat techniques.
Typically, it combines the two universes by having characters interact. Naruto characters might find themselves in the magical world of Winx Club or vice versa. They would then have to adapt to the new environment and rules. But as for the 'lemon' part which might not be suitable, we should focus on the positive aspects of this combination like how Naruto's combat skills can be compared and contrasted with the Winx Club's magical battles.
Many fanfictions also blend the storylines. The fight against the Empire in 'Star Wars Rebels' can be compared to the fight against Sauron in 'Lord of the Rings'. The rebels' struggle for freedom and justice can be seen as similar to the Fellowship's quest to destroy the One Ring. This way, the two universes are combined in a way that creates a whole new and exciting story.