Read through your short story. Write down a list of important themes, characters, plot points, setting elements and symbols in the story. Use a notebook and pen for this, or type your ideas directly into your word processing program. Read the story again. Draw a line through passages that are only descriptive and do not tell the reader anything about the important story elements you just listed.
Write down passages that are important but aren't quite translating to a script format. For example, for a long descriptive passage of highly symbolic value, you could turn the passage into a sort of internal dialogue for one of the characters, and that character could be thinking aloud about, or remembering, whatever the passage described. If you find a narrative passage that you can't lose, assign it to a "narrator" character.
Run a line through non-essential phrases in long soliloquies. Read aloud through all the work you've done on the story, and make sure it still makes sense and mostly reads the way it did before; it should only feel condensed and more dialogue-heavy, but not completely changed.
Type your new script into your word processing document. Print out copies of the script you adapted. Make enough copies for every participant to have one from which to read.