Begin by creating an outline. Write down the main events, character interactions, and the climax of your story. Then start writing the first chapter. Don't worry too much about making it perfect at first. Just get your ideas down on paper. As you write, you can go back and revise, adding more details and making the story flow better. You can also research more about both Baki and the other universe you're crossing with to add authenticity to your fanfic.