To start writing 'basement yard fan fiction', think about what makes basements and yards interesting. Basements can be dark and full of old things, and yards can be full of nature or have a unique layout. You can base your story on these elements. For example, if you're into a sci - fi fandom, the basement could be a secret laboratory and the yard could be the testing ground for some alien technology. Also, establish a conflict or goal for your characters. Are they trying to solve a mystery in the basement? Or are they trying to protect the yard from something? Once you have these elements in place, you can start writing the story.
Begin by brainstorming ideas related to basements and yards. What emotions do they evoke? Basements can be spooky or cozy, and yards can be peaceful or wild. Use these emotions to shape your story. Next, consider which existing fandom you want to incorporate it into. If it's a fantasy fandom, the basement could be a portal to another world and the yard could be the entrance guarded by magical creatures. After that, outline your story. Decide on the beginning, middle, and end. Maybe start with a character discovering something strange in the basement that leads them to explore the yard further. Then, as you write, let your creativity flow and don't be afraid to deviate from your initial plan if a better idea comes along.
First, you need to decide on the genre. If it's a mystery, you could make the basement hold some hidden clues and the yard a place where strange events related to the mystery unfold. Then, create your characters. They could be people who live in the house with the basement or neighbors who are curious about the yard.