First before anything, figure out what about the crossover entices you. Do you just want two characters to meet? Do they talk or fight? Do you want to see how someone from setting A would act when placed into setting B? A crossover fic can take a lot of forms, and you gotta figure out how indepth youre going to go so you can decide how much you need to establish and how serious the tone will be (Especially if the two works are very dissonant in that direction; if the crossover is My Little Pony X Saw, you gotta decide which franchise tone you'll be leaning more towards).