The simplest dry is nothing more than a black hackle wound from the hook bend forward to the eye. No joke, these things work incredibly well. They don't at all look like the real thing but it's not what you see it's what the fish sees.
Take one hackle (barbs extending just below the hook tip) and wind forward and tie off about 3-4 turns before the eye. Then tie on a slightly larger hackle and make 2-3 turns and tie off.
As FI said, these things flutter and skitter across the stream's surface and movement/life is generally the trigger for the trout to take. Because the fly is fully hackled and the eye rests higher than the rear, it rides high on the surface and is easy to skitter across the water without sinking it to create the skittering movement of the real bugs.
However, not all real bugs skitter 100% of the time and many times the bug will sit still on the water for a few seconds BUT the wings are still fluttering and this flutter, when viewed from underneath by the fish, appears as a blur and guess what, the wound hackle body creates the same blurry illusion of wing flutering.
It's all about creating the sight pattern that the fish sees and that's what this stupidly easly fly creates.