Film must be entertaining and draw an audience, but carry a good moral message - either subtle or obvious.
Here's your simple assignment: As an amateur non-screenwriter, WRITE THE PERFECT MOVIE!
Those guidelines are hilarious! But they are also beneficial. Short films are also deceivingly difficult to write. Every good film tells a story; short films are no exception. Before you can write your movie, you need to have a story. Make it easy on yourself and come up with a simple story. You've got a location and a character, so begin by asking yourself some simple questions:
1. Who is the old man?
2. Who are his closest relatives?
3. What is he doing in the basement?
Your movie has to begin with the old man in one moral or societal position and end with the old man in a different moral or societal position . Something happens (the major action of the film) to make him change from the first moral or societal position to the second. What happens can be figured out by answering the questions.
Example:
1. The old man is Jack Perkins, an elderly ex-boxer
2. His only surviving relative is his no-good nephew Vince
3. He is burying Vince underneath the basement steps because he learned Vince was trying to kill him for his life insurance policy
Figure out how to tell a story around these answers visually at first...don't use words. You can always add words later. Most short films are way too wordy...it's hard to condense a lot of backstory without using dialog, but try it because most of the backstory is inconsequential or can be filled in by the audience.
For instance:
The film opens with an image of the insurance documents strewn about the desk. Obviously there was a struggle because of the upturned furniture, overturned file cabinet, etc. We hear the shovel or pick-ax chipping away at the basement concrete and follow it down the basement steps where we see Vince's dead body. We see the old man struggling with the weight of his tools. Finally the police arrive, responding to the noise and arrest the old man. He is carried away as the credits roll. He started the film vindicated that he prevented his own murder and ended broken and busted. No dialog, just visuals. You can spice it up by having him try in vain to describe why he killed Vince as they haul him away, if you feel its necessary.
OR
Maybe the movie starts with the previous event: the old man discovers his insurance policy has been rifled with, Vince jumps him, and the old man pulls a pepperbox out of his sock and shoots him. Then you cut to the shovel scene as the credits roll. The old man starts as a benevolent, elderly uncle who included his no-good nephew as his sole beneficiary and ends as a murderer.
Figure out who the character is and what he is doing in the location. Come up with a story. Figure out where the character is when the story begins and where he is when the story ends. Figure out what sort of conflict causes this change. Do it simply, removing anything unnecessary to the duty of telling the story. And there's your movie.
Keep in mind, this is all probably as difficult as a lot of aerospace engineering, so good luck and have fun!