A few years ago we would not have had a good answer for this question, but having finally got back into fandom writing and really dug into it, I can tell you with clarity the definitive answer.

Start with the scene that you’re most interest to write.

You don’t have to write an entire epic story just to get to the few parts that make it worth it.

The audience doesn’t need you to hold their hand and explain all the lead up– people LOVE drabbles, and single scenes and short stories. As long as you give them just a little context to get a foothold on what’s happening, they’ll be fine. You can catch them up on what they need to know in an author’s note or even just a few bullet points.

I can not tell you how much easier and more exciting it is to write stuff when you don’t worry about how it all fits together. Snapshots are fine– wonderful even!

It’s okay to treat writing fic like a comic, or a piece of fanart that’s just one scene that expresses a moment that you want to share.

I’d start by writing the most romantic moment you can think of between your OC and their lover.

Often, I find once I get started with a scene, I’ll be moved to create more context and flow from there. There’s nothing wrong with starting “in medias res” and flashing backward, too!

I hope that answered your question at least a little! If I was unhelpful, or confused about what you were asking please let me know.