First, decide which American elements you want to cross over. It could be characters from different movies, shows, or books. Then, think of a unique premise or situation for them to be in. For example, if you're crossing over characters from 'Star Wars' and 'Back to the Future', you could have the DeLorean time machine end up on the planet Tatooine. Next, start building the plot around how these different elements interact and what challenges or adventures they'll face.
To start writing 'america crossover fanfiction', you need to do a lot of research on the source materials. If you're combining, say, American detective shows like 'CSI' and 'Sherlock Holmes' set in modern - day America, you have to know the characters, their personalities, and the settings well. After that, create an outline. Outline the beginning, middle, and end of your story. Maybe start with a mystery that only the combined skills of the CSI team and Sherlock can solve. Then, as you write, focus on making the crossover seem natural and interesting for the readers.