To start writing 4chan fanfic, first, you need to be really familiar with the content on 4chan. Know the different boards, the popular trends, and the characters or concepts that are often talked about. Then, just pick a topic you're passionate about from that and start creating a story. You can start with a simple idea like 'what if this character from that 4chan - related thing did this instead?'
Getting started with 4chan fanfic writing isn't too difficult. Begin by exploring 4chan and finding something that catches your interest. It could be an inside joke, a particular user's post, or a recurring theme. Once you've identified it, think about how you can expand it into a story. You might want to create a character based on a 4chan persona or use the unique language and culture of 4chan in your writing. Also, don't be afraid to be creative and add your own twists to the existing ideas.
If you want to start writing 4chan fanfic, the first step is immersion. Spend time on 4chan, observe the conversations, the memes, and the overall atmosphere of the different boards. This will give you a wealth of ideas. After that, decide on the genre or type of story you want to write. Are you interested in a comical take on a 4chan event? Or perhaps a more serious, emotional story? Then, start outlining your plot. Consider the characters you'll use. They could be inspired by real 4chan users or be completely new creations. Remember, the key is to have fun and let your creativity flow while still staying true to the essence of what makes 4chan interesting.