First, get to know the Subway Surfers world well. Know the characters, the setting, and the basic rules of the game. Then, think about the two male characters you want to feature in your fanfic. You can start with their backstories, like where they come from and what led them to subway surfing. After that, plan out how they meet and how their relationship will develop. For example, it could start as a friendly rivalry and turn into love.
When writing a 'gay subway surfer fanfic', it's important to do your research on subway surfing if you're not already familiar with it. You can watch videos or read about real - life subway surfing experiences. For the gay aspect, you need to be respectful and sensitive. Build the relationship between the characters gradually. Start with their initial interactions, which could be based on mutual admiration for each other's subway surfing skills. As the story progresses, introduce elements that deepen their connection, such as them opening up about their personal lives. And always keep in mind the overall theme and mood you want to convey in your fanfic.