To start writing Thrawn fanfiction, you need to immerse yourself in the Star Wars universe. Know the rules, the technology, and the different species. For Thrawn specifically, understand his Chiss heritage and how it influences him. Start with a simple concept, like Thrawn on a diplomatic mission. Build your story around that. Don't be afraid to add your own twists and turns. And remember to keep his character true to what we already know about him.
First, study Thrawn's character. Read the official books and watch the shows where he appears. Get a sense of his personality, his strategic thinking, and his goals. Then, come up with an original idea. It could be a 'what - if' scenario, like what if Thrawn had to face a new enemy. Next, create an outline for your story. Decide on the beginning, middle, and end.
Begin by brainstorming. Think about different situations that would be interesting to put Thrawn in. Maybe he's stranded on a strange planet or has to deal with a mutiny on his ship. Once you have an idea, research the Star Wars lore to make sure your story fits in. When writing, focus on Thrawn's intelligence and his ability to read his opponents. Use descriptive language to bring his character and the Star Wars setting to life. Also, get feedback from other Star Wars fans to improve your fanfiction.