Place a fly attractant inside a masonry jar. Attractants are sold at some gardening stores, often including housefly pheromones. Do-it-yourself attractants include old fish, meat and other protein waste, as well as fermented molasses or stale beer.
Fill the jar with an inch of soapy water. Once lured into the jar by the attractant, the soapy water traps the fly. Soap reduces the surface tension of water, drowning the flies when they get caught in it.
Secure a piece of paper to the top of the jar using a rubber-band.
Cut a dime-sized hole in the center of the paper. Flies are lured through the hole by the attractant's smell, but they find it difficult to navigate back out of the hole after feeding. This keeps houseflies trapped in the jar longer, increasing the likelihood of drowning in the soapy water.