First, you must choose a crowded public place, such as a restaurant, party, exercise center, sporting event, or concert (or wherever people gather in a crowd.
You must eavesdrop on a conversation between two people you do not know (as unobtrusively as possible), and you must record enough of the conversation in your memory to use later on.
As soon as you are able, write out as much of the conversation as you can. You should at least be able to record five to ten lines with a fairly high degree of accuracy, but don’t worry about being totally accurate.
After writing out the conversation as a bit of dialogue, list as many descriptive details as you can about the two interlocutors. Try to get down their something of their personality by describing their appearance, manner of speech, actions and reactions, and—of course—words. Obviously, you will probably have to expand it and/or embellish their conversation
Your specific task is to write a story out of the two characters and their dialogue. You may, if you want, choose one of the characters and imagine yourself as that person; you would then write from a first person perspective. If not, then you can describe both people from an omniscient third person perspective. Create a story that will demonstrate in a specific context how the two characters came together and what their conversation means. Make the two characters and their conversation come alive.