Well, the first step in writing convergent fanfic is to do your research on the relevant fandoms. If you're into a video game fandom, study the different levels, characters' abilities, and the overall lore. Once you've got a good grasp, pick the elements you want to converge. It could be combining the abilities of different playable characters. Then, create a conflict or a goal for your story. Develop the relationships between the characters from the converged elements and start building your story around that. Remember to stay true to the essence of the original works while adding your own creative twists.
First, you need to be well - versed in the fandom. Know the characters, the settings, and the different storylines. Then, start by identifying the elements you want to converge. For example, if it's a 'Harry Potter' convergent fanfic, you could choose to merge the magic systems from different wizarding schools. After that, come up with a basic plot idea that can incorporate these converged elements.
To start writing convergent fanfic, begin with a deep understanding of the source materials. Let's say you're in a fantasy fandom. You should study the different races, magic systems, and cultures. Next, think about how you can bring together different aspects in an interesting way. Maybe you want to combine the courtly intrigue from one series with the epic battles from another. Outline your story, develop your characters based on the combined elements, and then start writing your narrative.