Decide on your key word if you have not been given one already as part of a writing exercise. If you are struggling with this, open a dictionary at random and pick the first noun you see.
Write the key word down in the center of a piece of paper in black ink and note around it all the immediate ideas and images it brings to mind.
Look the word up in the dictionary and write down its definition in green ink, at the bottom of the piece of paper. Compare the dictionary definition with your own ideas about this word. Write down with green ink any words the definition brings to mind.
Look up the word in a thesaurus, and write down in red any antonyms (words which mean the opposite) it suggests. If these inspire any ideas about your key word, write those down too, in green ink. Also make a list in blue pen on another sheet of paper of words the thesaurus lists as having the same or similar meaning (synonyms). This will be useful to you later in the writing process.
Re-read everything you have written down on the first piece of paper. Decide whether you wish to focus the story on one of your original ideas or one inspired by the dictionary definition or antonyms -- any one or a combination is fine. Spend as much time as you need thinking about this until you have the basic idea for the story.
Take a fresh sheet of paper and write the key word at the top in large letters. This will be your principal writing prompt. Underneath this, sum up the basic plot of the story -- the beginning, middle and end -- in no more than three sentences. Keep it short as that will keep you focused. List all the story's characters, key objects and events with a sentence beside each of them summing up their main characteristics and vital points. Keep this short and to the point. Detailed character development should be done on separate sheets of paper if desired.
Place the principal sheet in easy view and start to write the story. Refer back to the sheet as necessary. If you run out of ideas, return to your other notes. Your original ideas (written in black) are likely to be the most help, but secondary ideas (in green) will also inspire you.
Keep writing until the story is finished. Take a break, then re-read it, noting any parts you think could be expressed better, don't make sense or need additional writing. Make these changes and keep re-writing until you are happy with the finished story.