To write a great fanfic about Jason being badly hurt, you need to research. Look at how other fanfics handle injuries. For the story, start with a strong opening that shows the moment of injury vividly. Then, develop the plot around his recovery. Include his friends' reactions, like Annabeth's strategic thinking on how to help or Percy's concern. And don't forget to add some internal monologue for Jason to show his pain and determination.
Well, when writing a fanfiction about Jason Grace being badly hurt, think about the theme you want to convey. Is it about sacrifice? Or the fragility of heroes? Plan out the details of his injury carefully. Is it a slow - building ailment or a sudden, catastrophic one? Also, use descriptive language to convey his pain. For example, 'Jason felt as if every nerve in his body was on fire as he lay there, badly hurt from the fall.' And build the story around how he copes and how his friends support or fail to support him.