To make the fruit fly trap, you'll need an empty water bottle, apple cider vinegar, and a box cutter. Using the box cutter, cut the top of the water bottle off, just after the bottle's taper ends.
Pour about an inch of apple cider vinegar in the bottle.
Invert the top of the bottle and place it into the opening you previously cut.
If the top slips down into the bottle, or has any small openings along the edge where it doesn't quite seal, then use clear packing tape along the top, cut edge to make sure there is a seal and to hold the inverted top into place.
You could empty and reuse the same fruit fly trap, but it seems cleaner and easier to just toss that one when it needs freshening and start with another empty water bottle.
It's strangely fascinating to check the bottle every day and see how many more flies have been trapped. Fascinating and satisfying.