Well, I don't think, there's a big difference between writing fiction and Fan fiction, so I would recommend:
1. What kind of story (retelling your own game or just a story of the Valley...)
2. Who is the MC, who's the Antagonist? Create interesting characters and plausible conflicts.
3. Romance included? Smut included?
4. Fan fiction should mainly follow the game canon, changes or developing should be possible to happen in the game, too.
5. As for writing in general: read, read, read... I know, there's many fanfic on Wattpad, for example, or here in the forums. And analyse the text by asking yourself, e.g. why you like it or not or what you would do different.