One way to start writing in this genre is to focus on the emotions. Think about how the male character would feel having a symbiote attached to him. Is it fear? Curiosity? And for the symbiote, what kind of emotions does it have towards the male? Once you have a clear idea of the emotional aspect, you can start building the plot. Consider different scenarios like them going on adventures together or facing challenges as a couple. You can also look at other yaoi fanfictions for inspiration, but make sure to add your own unique twist.
To start writing Symbiote x male yaoi fanfiction, begin with creating a strong backstory for your characters. Why did the symbiote choose the male character? What are the male character's initial feelings towards the symbiote? Also, don't be afraid to explore the unique aspects of their relationship. For example, how does the symbiote's presence affect the male character's daily life? You can start with a simple scene, like the moment they first meet, and build from there.