Although two hackles were in the original recipe I believe Vince Marinaro himself said if you could find a hackle of suitable length one would suffice. With todays hackle length is usually not a problem.
Tie in your hackle behind and at the base of the wing and dub a small but very tight ball at the base of the wing with half the ball in front and half in back. I prefer synthetic dubbing for this as it seems to make for a better anchor for the hackle. Wind the hackle diagonally you want it to sort of wedge in the corner where the dubbing ball ends. Two turns front to back two turns back to front. Hope this helps. If this seems confusing let me know and I'll try to explain it in a different way. By the way this style of fly is my absolute favorite.
Turk