Roll paper up like a funnel with a tiny hole in it and tape it. Pour vinegar into the jar to cover the bottom. Place funnel in and tape around the rim so no flies can escape. I cut down the paper down so it's not so big. Set the jar in your fruit area and watch them be attracted.
When the jar gets full of flies take them outside and set them free. Unless you want them to die than you add a drop or two of dish soap and they will drowned.
I have no fruit flies in my jar because it works so well. Actually there are two in there you just can't see them.