You will need:
a piece of paper
a piece of tape
some dish soap
Make a little funnel out of your paper, simply by rolling it up to a dime sized point on one side (fi you look at my picture, you can see what I mean). Tape in place with your piece of tape.
Now, put about a quarter cup of water in your cup, followed by a quarter cup of vinegar (we used cider vinegar, just because that was what was on hand) and a teaspoon of dish soap. Mix together gently.
Place your funnel on top of the glass, and viola! I keep mine next to our fruit bowl, and you will see that in a few hours time, the liquid will be filled with with flies (yuck! But better than circling around your fruit), and within a day, the fruit flies will disappear all together. Definitely one of my favorite natural home fixes.