All you need for this is a wide-mouthed jar, some sugar water, and a stiff paper cone. Pour some syrup into the container as bait. Next, fashion a medium-length paper cone, making the wide end larger than the jar's opening and cutting a hole an inch in diameter at the apex (see Drawing No. 1). Set the cone upside down in the mouth of the container, perhaps taping it in place. Now, put the jar in some unobtrusive spot, and let it be. The flies will be attracted to the syrup and will go down through the cone into the glass trap. Once inside, they won't be able to fly up through the 1" hole to get out again. All you have to do is check, and occasionally clean, the jar.