BOY, this stinks !
Fill the bag to the mark with water, and hang it up where the flies are causing a problem.
Just don’t think you can use it indoors, or on your patio (despite what they claim on the packaging), or in fact anywhere within smelling distance, because it gives off a rotten, sickening smell, something like dog poo but worse.
Flies, however, love the smell, and they merrily head right inside the bag to find out what is giving off the pong. Once in the bag, they land in the water, and drown (probably making the smell even worse as they rot).
It’s a simple, and crudely effective product, and as long as you place it downwind, it’s great, but don’t say we didn’t warn you about the smell….