Use a structural template to fit it in

If you're into some structure in your writing, I'd suggest using a structural template like a Story Circle, and then using that to find a natural spot for your bit of conflict in some broader narrative.

The Story Circle is a simplification of the Hero's Journey as specified by Joseph Campbell, further developed by Dan Harmon (of Community/Rick and Morty fame): it splits stories into 8 themed chunks that when put together make for a narrative that is easy to follow and satisfying to read or see or hear. It's a simple template for telling any kind of story in a way that is immediately familiar to most people.

The parts of the circle are shown here:

Each part of the circle can also be subdivided into a smaller, "nested" story circle, if your target length demands more content for the overarching story. I've had success fitting similar fragments of narrative into larger projects by looking at what part of the circle the particular story fragment naturally fits into.

For example, sections 3, 4 and 6 often contain conflict (especially the hero getting defeated in some way in 4 or 6) so those could be good spots to try to fit in a fight scene.

For more information, there is also a simple video on YouTube from adult swim where he summarizes it.

When I come to a problem like this I usually write a list of all the characters, ideas, scenes and other things I want to include in my story. Like this:

• Include a battle scene between _____ and _____.
• Add a character called ______.
• Use a twist were _________.

And so on...

Anyway, after I've done that I try and piece them together like a puzzle as Rolfedh said before. If I can't think of a good way to piece together some ideas I would:

1. Look at how other authors use the idea.
2. Take a break.
3. Get input from a family member or a friend (two brains are better than one!).

I hope this helps but if in the end none of this works out I suggest slightly altering the idea/scene/character so it's still slightly like the first idea but different. Example: A fight scene with magic becomes a fight scene without magic, a physical war between two enemies becomes a verbal one.

Do not worry, give it some time.

If you have what seems to be a good idea, your mind won't let it go. Eventually you will either build a story around it, or, while working on some other story, you'll discover that the new story and old idea would actually go pretty well together.

Don't settle for a bad story around a good idea. Let it work through.

Don't toss them out; collect them!

What you've got there is a story fragment. You can start a collection of story fragments. Chances are, your fragments are a constellation of related ideas. As you write and collect them, you might start seeing connections between them. Then you can write those in-between connecting parts and assemble them into a larger story.

What I do is write the idea itself. By the time I have done that a solution or the next idea usually suggests itself. If it doesn't, I go for a walk and try putting random ideas or words together to find a solution.

In the rare case that these don't work, I revise something else I have written and then come back to the piece. At this point I sit there until I have something sensible to write, or, very rarely, decide that the idea can't be progressed.