Okay, let me see if these directions help you. I'm assuming(yes, I know that's always a mistake) that you know the recipe and have the correct materials except perhaps one. The typical recipes are IMHO, incorrect as far as the hook. You may read that you use a 'standard' dry fly hook. Wulff, as did Dan Bailey, tied this pattern on a 2xl hook. I believe Wulff used the mustad 9671 or even the 9672. I'm guessing that the longer hook was to approximate the actual length of the insect imitated(Green Drake) and so the materials could be applied in better proportions. As I sit here right now I'm not sure if a size 8, 10 or 12 was used.

Tie tail in. Bring thread to thorax area. Prepare clump of bucktail or calftail and tie in about 2 eye lengths behind eye. Split clump with 'figure 8s' to create wings. I additionally encircle each wing 2x and manipulate into the position I want. Place 1 wrap directly in front of wings and 1 behind wings. If the wing is too far forward,push it back with your thumbnail. Bring thread to mid abdomen. Apply dubbing to thread leaving a little bare space at top of thread. Wrap to tail and when dubbing starts, start wrapping to the wing. Stop about 1 turn from the wing. Take 2 or 3 Silver Badger hackles and tie them in with the tips pointed towards the tail and the butts to the eye. They should be tied down so that the one with the heaviest stem is on top and wrapped first. Begin wrapping 1st hackle forward, leaving a little space between each wrap. Use an equal number of wraps behind and in front of the eye. Better to err on the side of less wraps. Tie the hackle down. Begin wrapping the 2nd hackle going through the wraps of the 1st. Tie down at head. Do the same if you've used a 3rd hackle. Trim the hackle and complete with a whip finish.

Hope this helps.

edited PS: I meant to mention that the hooks I like for this, and similar large flies, are the Mustad 94831(2xl 2xf) and the Daichii 1290(2xl).

H.