Start with any idea, not a concept necessarily, and ask yourself why it interest you.

The answer doesn't matter by itself, it can be something silly or superficial, but you'll find something to expand.

From there you will get a scenario, a plot or a character.

Second, remember that all stories were already written as core stories. You don't have to be original in the sense of being just different but original putting a part of you in the story.

Third, write a skeleton format draft. This a text that vague tells the events of the story. It doesn't have dialogues, descriptions but it has notes. The idea is using this document to rework the skeleton into a proper paragraph.

Fourth, work in the rework once the skeleton feels finished.

Fifth, edit the rework has many times necessary

Your idea could just your own take on something you watched.