What do you want your fly to do? Sink deep or pop on the surface? Jig or swim straight? Zig zag up and down or back and forth? In streamer design these are necessary questions to ask. Incorporate materials that will allow for what you want to do, and shape them is such a way that in the water they will move just right. Then as Kelly Galloup says, go out and see if the fly hunts.
Get it right and maybe you'll entice a big old paddle tailed brown to eat a clump of chenile and feathers.