Gather the supplies: Empty Spaghetti Sauce Jar, (Or any other discardable container on hand), Produce, Balsamic Vinegar, (Any Vinegar will work), Plastic Wrap, Rubber Band, and Toothpick.
Cut up Produce, place in Jar. If the Produce is already starting to decompose, even better. Use the produce that has attracted the gnats/fruit flies.
Add Balsamic Vinegar. The Gnats/Fruit Flies are attracted to the vinegar and get stuck in it.
Only need enough vinegar to cover the bottom.
Cover top with Plastic Wrap and secure with Rubber Band.
Poke small holes in Plastic Wrap with Toothpick for Gnats/Fruit Flies to enter.
Label clearly. Place in area Gnats/Fruit Flies were seen. Leave till no longer see the pests flying around. Toss your trap in trash.