The credit for first designing variants goes to William Baigent of Yorkshire England. Preston Jennings dedicated a chapter in his book called: A Book of Trout Flies, to variants. Art Flick also discussed and showed dressing for variants in his book called:, New Streamside Guide. The Catskill fly tiers really helped bring awareness to variant dry fly patterns, such as: Grey Fox Variant, Dun Variant, Cream Variant and Blue Variant. The biggest issue today with tying large variants is finding hackle that is good quality, long rooster hackle.
Variants are really just an oversized hackle wing, a short body and a tail. For variant dry flies normally the hackle is 1-1/2 to 2 times longer than the bend of the hook. For example, for a size #14 hook I like to use hackle that is really for a size #10 or #8 dry fly. The hook can be any dry fly hook you prefer to use, but in the appropriate size.
The key to these patterns is finding rooster feathers that are both long enough and stiff enough. I like to use 1 to 2 feathers for the wing section. I usually strip the barbs off the bottom of the feather before securing the feather to the hook shank. For the bodies I prefer to use stripped peacock quills or gold tinsel. Although, I am not sure what you use for the bodies really matters since the bodies are so small and the flies ride so high in the water.
For slow moving pools or the shallow tail sections of a pool I prefer to use just one hackle. That helps the variant fly pattern to land delicately so not to spook the trout. For fast runs I use two feathers to help the fly float better in the faster water.
The tail fibers are Whiting Coq De Leon tailings. (I do not like to use the Whiting Coq De Leon rooster capes or saddles for tailings because I feel they are not stiff enough.) The best tailings I can find today come from the shoulder area of the roosters. Personally, I don't think the color of the tailing fibers really matters to the trout, just to the fly tyer.