How do you make the best crossover?
So, here's the question: I want to do a crossover between Delicious in Dungeon and Countryhumans. Countryhumans are being hunted by humans and are being hunted down, and one of the main characters, my OC, Haruto (a ninja from the mountains to the east, where Shuro the samurai comes from), finds one of the Countryhumans as a child after leaving the party after losing his right eye to the Red Dragon. I've already gotten that far with the idea. Are they allowed to take part in a crossover, and what do you recommend?
Forward: I don’t know any of the universes you want to link. But there are some general needs that all crossover stories must follow.
Thank you.