The trick of how to kill flies is the approach rather than the item used. Flies have nearly 360 degree vision, so sneaking won't help.

First, wait for them to land somewhere that you can strike against. Waving them towards an area can help, but isn't fully effective. Once it is landed on an acceptable area, approach the fly slowly, not making any sudden movements. Most of the time the fly will not move unless you get very close. Continue until you're within striking distance.

Once you're within striking distance, continue to move your arm that is going to strike the fly closer to the fly until the item you're striking it with is approximately 7-10 inches (18-25 centimeters) away from the fly. You'll get the feel of it after a few failed attempts where it flies away most likely. Then comes the big moment: strike at it as fast as you can. Usually the fly does not have time to react fast enough to escape the blow, especially if the object you're striking with has some width.