Place the fruit in the bottom of the cup. I've used 2 blueberries or a couple of lemon slices. Whatever you have around works fine, and if it's kind of spoiled, that's even better. Pour the balsamic vinegar (you could also use cider vinegar or another sweet vinegar, but NOT regular white vinegar, it won't work) over the fruit. Place the plastic wrap tightly over the cup so that it seals it. If it's not airtight, put a rubber band around it or tape it to the cup. Poke a very small hole in the plastic wrap so that the flies can get in but not easily find their way out. Let it sit for a few hours and it will attract all the fruit flies. it works really well if you leave it overnight. Then, take the cup outside, remove the plastic wrap, and let the flies free! This has worked really well for me the couple of times I let some food sit in my trash can when I left for the weekend and forgot to take it out before I left.