Place piece(s) of fruit-melon/banana/etc-at the bottom of glass jar. Create a cone out of paper, balance on rim of jar so the opening at the pointy end is just big enough that a fruit fly can get through & does not touch the fruit at bottom of jar. (See more info for rest)
The fruit flies will get into the jar through the tiny hole but won't be able to get out. After you've completed your fruit fly trap, wipe down ALL surfaces (counters, appliances, etc) with white vinegar. Leave your trap overnight. It's shocking to see them all in the morning stuck in the jar!