If you end up with a draft of a short story or a few paragraphs of a typical UX interaction scenario, or a storyboard, or a little film of someone swiping on a screen to show how your App idea would work — you have not done Design Fiction.
What you’ve done is write a short story, which can only possibly be read as a short story.
What you should ideally produce is something a casual observer may mistake for a contemporary artefact, but which only reveals itself as a fiction on closer inspection. It should be very much “as if..” this thing really existed. It should feel real, normal, not some fantasy.