One possible story is:
I had 5 apples. My sister gave me 7, and now I have 12.
Explanation:
We are writing a story for 5+7=12. This means we need to start with 5 objects and somehow gain 7 more. In our story, we start with 5 apples and we get 7 more.
The last part is that our total must be 12. In our story, we end with 12 apples.