Prepare the jar. Secure the metal lid on the jar. Using a metal skewer (or small nail and hammer), carefully poke holes in the lid. The holes should be large enough for fruit flies to fit through.
Place ingredients into the prepared jar and stir until the honey is dissolved. Top with the lid and set on the countertop near fruit. The flies are attracted to the scent of the sweetened vinegar.
Change the ingredients in the trap weekly if you have flies, or leave on the counter for several weeks, as needed.
Notes
Any vinegar can be swapped for apple cider vinegar.
Cane sugar can be used in place of honey.
No need to replace the contents of the jar if you aren't actively getting flies. This can sit out for several weeks before needing to be replaced.