First try to organize the structure of the story so that it is clear for yourself, using perhaps the same one in the requisites of the exercise.
This first piece of advice can be used to any of the elements of the story that you may be preparing: <em>detail everything to the point that you feel you can write the story off them</em>. This can change mid story, or you can come up with different or more ideas, but its good to have a solid ground so your stories can take off safely.
The first requisite is to set up the story, so define in general lines each of the details listed in the exposition paragraph. To make matters easier, it's preferable that the problem or drama central to the story is defined at the very beggining, even before the setting and characters themselves are defined. This is because it's easier to "move" the storylines to the direction you'd want them to go, when you know where it is supposed to go.
<em>The basic plot structure is the way to go here: start with a conflict or situation, put your characters in contact directly or indirectly with it, put obstacles in their way and lastly put a resolution to the conflict. </em>
The story need to have multiple lines, so define really well how each detail interact with each other before you start. This a short story, so maybe its better for you to use only one or two parallel storylines. Maybe a side villain or unsuspecting person is acting or suffering interference of the main plotline. And how or when this connects to the main plot it's up to you.
<em>Style and Conventions: describe the emotions, feelings, the places and events and how your characters react to them in the most concise way. You don't need a lot of words to say that a character is feeling cold and that it makes him/her remember of another time or simply makes them miserable, for example.</em>
It's asked of you that the conclusion provides a reflection or resolution, so this is the time for the mystery to be revealed, tragedy to be stopped or love to reach its peak. Maybe you'd want to write how the characters of the story are affected by the closing of the conflict.
