First, you need to do your research on both universes. Know the characters, their powers, and their personalities well. For the harem part, decide which characters will be part of it and how their relationships will develop. Maybe start with a plot device like a magical object that links the two worlds. Then, gradually introduce the characters to each other. Write engaging dialogues that show the different cultures of the two universes.
To write this kind of fanfic, think about the themes that are common to both Harry Potter and the Justice League. It could be about justice, friendship, and sacrifice. For the harem aspect, consider the dynamics between the characters. You could have a character from Harry Potter be the center of the harem, or vice versa. Build the relationships slowly, through shared experiences. For example, they could fight a common enemy together. And don't forget to add your own unique twists to the story. Maybe there's a new magic spell that can enhance the superpowers of the Justice League members. Write with passion and let your creativity flow.
When writing a Harry Potter and Justice League harem fanfic, start by creating a believable scenario for the crossover. It could be that a dark wizard has found a way to enter the Justice League's world and is causing chaos. Then, bring in the characters. For the harem, it's important to make the relationships feel natural. Don't just force characters together. Let their personalities and the situations they're in drive the relationships. You can also play with the different magic systems and superpowers. For instance, Wonder Woman could learn some basic spells from Hogwarts, which could add an interesting layer to her character. And always keep in mind the overall story arc and how the harem relationships will evolve throughout the story.