This is not my design, I found it online and was asked to explain how I did it. If I could recall, I would credit the person or organization who shared the idea. I cut through (like a cross-section) a two liter clear plastic soda pop bottle about a third of the way from the top. I turned the top (no cap) upside down and slid it into the bottom until it was just snug. I put old wet cat food and an inch or two of water in the bottom. It took a few days to get going, but flies were drawn to the smell and drowned. The more flies that drowned, the more came. I empty it when it seems full into a compost pile a little way from the house (it really is whiffy when you are holding it near you). I refill the bait and let it go again for some weeks. The tricky part for me is getting the bait just right. I tried duck droppings but it did not attract nearly as many flies as rotting meat. I place it out of reach of children and pets, but not far from the duck pen.