This really is an amazing nontoxic fly repellent and it is incredibly easy to do. In fact, one day several years ago, my wife and I saw this at an outdoor restaurant and had to ask what it was.
All you have to do is take a sturdy Ziploc beg, preferable a freezer bag, and fill it about half way with water and you can add some sugar, or some people even add a penny to increase the effectiveness of this neat little trick. It works on the principle that flies have thousands of eyes and the magnification of the water messes up their sight so they stay away from the bag.
The sugar in the water helps refract the water better so it really messes with the flies’ vision. I’m not sure about using the penny, though that’s what some people do. This only works with flies and not other flying insects.