Writing the first draft of a short story is distinctly different from writing the first draft of a novel. Novels ten to require a great deal of forethought and planning. Additionally, each progressive chapter locks you further into the plot, limiting your options as you get closer to the end. With a short story, you don’t have those worries. Lengthwise, a short story is similar to the first chapter of a novel. If you don’t like how it is going, you can scrap the draft and move on, without worrying about complying with a series of expectations or requirements.
Writing the first draft of a short story is about exploration. You start with a plot or character or conflict in mind, and you explore it for as long as it seems interesting or relevant. Once you reach a sufficient conclusion, you stop. This may take you ten pages or thirty pages or more, but it doesn’t stretch out in the way that a novel does. Some people can write the first draft of a short story in a single sitting. That may not be your process, but with a short story it is certainly possible.
The overriding advice that I can give for a first draft of a short story is to relax and let the writing take whatever course it will take. Cleaning up and revising a short story isn’t nearly as complicated as doing the same for a novel. A short story is short enough that you can even throw the whole thing out and start from scratch if you don’t like your initial results. That’s the sort of freedom a short story gives you. Take advantage of it.