Next, start planning your story structure. Decide on the main characters from each side that will be involved in the crossover. Outline how they will meet and interact. For example, if you're crossing 'Left Behind' with a science - fiction series, you might have a spaceship land in the middle of the post - apocalyptic world. Then, think about how the characters will react to this strange new element in their world. This will form the basis of your plot development.
Finally, just start writing. Don't worry too much about getting it perfect at first. Let your creativity flow and see where the story takes you. As you write, keep in mind the voices of the characters and the rules of the two universes. You can always go back and edit later to make sure the crossover feels seamless and the story is engaging.