First, know the characters well. Watch the movies again to understand their personalities, strengths, and weaknesses. Then, come up with an interesting plot. It could be something like a new adventure for Po and his friends, or a backstory for one of the side characters. Next, start writing and don't worry too much about grammar at first. Just get your ideas down on paper.
Writing Kung Fu Panda fanfiction can be a really fun and creative process. Begin by brainstorming ideas. What if Po had to face a challenge that was completely different from what he had faced before? Maybe he has to save the Valley of Peace from a magical threat. You also need to develop your characters further. Give them new quirks or relationships. And don't forget about the setting. Describe the Jade Palace or the surrounding areas in more detail. As you write, read it out loud to check for flow and make sure it's engaging for the reader. This way, you can create a great piece of fanfiction.
To write Kung Fu Panda fanfiction, you should start by choosing a theme. For example, you could focus on the theme of friendship like in the movies. Then, think about how to expand on the existing world of Kung Fu Panda. Maybe introduce a new villain or a new type of kung fu. Also, make sure to keep the tone consistent with the original. If it's light - hearted in the movies, keep it that way in your fanfic. You can also read other fanfictions for inspiration.