To write this fanfiction, think about the backstory. For example, if there has been a recent big battle and Tony has PTSD from it. He may feel like he can't handle the pressure anymore. As for the actual jumping part, use vivid descriptions. The moment his feet leave the edge of the building, the feeling of weightlessness. And when writing about Steve, show his love and panic. He could be across town but sense something is wrong through their deep connection. After the jump, you can either have a tragic ending or a heroic rescue, depending on the tone you want to set for your fanfiction.
First, establish the emotional state of Tony. Why would he be in such a desperate situation? Maybe start with his insecurities about his relationship with Steve. Then, describe the setting. Is it a tall skyscraper in the middle of a bustling city? Next, build up the tension before he jumps. Show his internal struggle. When he jumps, focus on the fall. How does the wind feel? What is going through his mind? For Steve's part, show his reaction when he realizes what's happening. You can also add in some side characters' reactions to make the story more complex.