I don't know the specific story they tell. It could be an adventure story about their exploration in a magical land, full of strange creatures and hidden treasures.
They might start by sitting down in a cozy place, like under a big tree. Finn could begin with 'Once upon a time...' which is a very common way to start a story.
I'm not sure specifically about Finn and Jack Harries gay fanfic. However, fanfic in general can be found on various fanfiction platforms. You might want to check websites like Archive of Our Own (AO3). They have a wide range of fan - created works, but it may take some searching to find something related to Finn and Jack Harries in that context.
Another option is to look at the related videos on YouTube. Sometimes, fanfic writers promote their works through video descriptions or even make videos about their fanfics. Although it might take some time to sift through, it could lead you to a great fanfic about Jack and Finn Harries' sister. Also, don't forget to check the comments section as fans might be discussing other fanfics there as well.
One common theme is adventure. Fans often write about Finn and Jack going on exciting journeys, like exploring uncharted islands or trekking through dangerous jungles.
Maybe they would tell a story about friendship. Since they are friends in real life or at least colleagues in the acting world, they could create a story of two friends going on an adventure together, facing challenges and helping each other out, like exploring a magical forest filled with all sorts of amazing creatures.
One common theme could be family dynamics. Since it involves their sister, exploring the relationships within the family, like how she interacts with Jack and Finn, might be a big part. For example, a story could be about her getting involved in one of their projects and how that affects their family bond.
Tumblr can also be a source. There are many fanfic communities on Tumblr. You can search for Finn and Jack Harries fanfic tags. However, Tumblr's search function isn't always perfect, so you might need to dig a bit deeper by exploring different blogs related to the two. Also, be cautious as the content quality can be inconsistent.
First, develop your characters well. For a 'fack' love story, you need to understand the personalities of the characters involved. Then, create an interesting plot. It could be a slow - burn romance or a love - at - first - sight kind of story. Also, use vivid descriptions to bring the story to life.
They might tell a spooky story, considering Finn Wolfhard has been in some horror - themed works like 'Stranger Things'. Maybe it's about a haunted house or a mysterious creature in a small town.