Some new writers try to pack a very long story in just a few pages. For example they will write about the whole life of a gangster starting from his birth and then finishing when he’s in his forties – all in three pages. This is covering too much, and means that a lot of interesting bits get left out.
Think about one of your favourite movies – no movie would follow one person’s life from birth to adulthood. They might show you scenes from different times, but would not give all the facts from babyhood, to primary school, to high school etc.
It is the same for writing. You can choose scenes from someone’s life to describe, but you don’t have to describe everything. It’s fine to leave out huge sections if you choose to. And for a short story, it is best to focus on a much shorter space of time, or one particular event. In this way you can make your reader experience your story with you. This links with Tip 5 – show, don’t tell.