Pour your juice or wine into your glass container. A couple of centimeters of liquid is all you need.
Mix in a couple of drops of dish soap.
Secure plastic wrap over the top of the container. Make sure it is tight. A rubber band may help keep the plastic wrap in place.
Using a bamboo skewer, poke some holes in the plastic wrap. Make sure the holes are big enough for the fruit flies to get through but don't make too many.
Clean the area so only the trap will tempt the fruit flies. Replace every 1-2 days as needed.