Take a glass and put some fruit-fly bait in the bottom. A piece of tomato or banana works well.
Cover the top with saran wrap and poke some decent-sized small holes in the top [just realized my holes have been too big — they should be tiny, made with a pencil tip or some such]. Use a rubber band to affix.
Put the trap on your counter and let the hunt begin. Every morning and sometimes every few hours subsequently I have to cover it with my hand and bring it outside to release the catch. (I feel kind of silly letting them go free, but it’s too much trouble to kill them). I find it kind of fascinating that the cup is often hot from the methane emitted by the rotting banana.
It’s gotten hot and sultry, and too many fruitflies buzzing around makes me feel like I’m in Baby Doll.
Ivan Kreilkamp 
The trap we’ve started using is an unfinished bottle of red wine The flies drown. I realize in most households “leftover wine” doesn’t exist.
I’m definitely going to do this when I get home today. Way too many fruitflies. Thanks Ivan!
Glad to be of service. Btw red wine does work very well too, maybe red vinegar too, not sure.
Ivan, thanks for reminding me of this … I used to make a trap with a paper funnel on top … but this was simpler and has worked well the last couple of days.
I’ve been doing the same thing recently only with a reusable plastic bag – instead of poking holes, I just leave it open and then come in to swoop it closed. It gives me the thrill of the hunt …
That does sound exciting, though I’d worry that too many flies would come and go in the meantime…
Leave the trap on the counter all day, After dark, with the kitchen lights out, cover it and move it to the freezer. In the morning, all the flies will be dead.