What you need: Apple Cider Vinegar, Dishwashing Liquid (two drops), a Bottle, and Plastic Wrap.
Okay, so you take a transparent bottle and pour in some apple cider vinegar (not a lot, just enough for the flies to drown in). Add the two drops of dishwashing liquid and stir it. Then cover the mouth of the bottle with plastic wrap. Make a few big holes on top. Leave it somewhere in the kitchen/dining area.