Start brown Veevus 14/0 about two-thirds of the way forward and leave a long tag end — that tag becomes the rib. Wrap back over it down around the bend and bump the thread so it can't slide.
Spin a noodle of tan Superfine Dubbing on the thread, starting thin and working thicker. Use the bare thread between the dubbing and the hook to get the first turn of dubbing to land right at the bend, then build a slim tapered body forward.
Counter-wrap the thread tag forward as a rib in evenly spaced turns — brown over tan gives a nice segmentation. Tie it off with a few turns and clip it out.
Underwing: peel a clump off a natural CDC feather and bundle it neatly. Make a thread base up to the hook eye and back to the front edge of the body, tie the CDC in on top nice and firm, and trim the butts out at an angle.
Even the CDC tips by holding them just beyond the bend and breaking them off with your thumbnail. Don't cut them square — you want that randomness.
Prep a soaked mallard flank feather: strip the fibers off the base, create a separation point between tip and butt along the centre stem and draw the fibers back. That point becomes the base of the wing.
Keeping the prepped feather flat — not pinched or rolled — lay it on top of the hook with the centre quill sitting right on top of the shank and anchor it with three or four turns. Lift the centre stem out, take a couple more turns over just the fibers, then trim the stem and the leftover fibers with the very tips of your scissors.
Split the wally wing: separate a few fibers on the far side and pull the main stem away from them so the feather skins down, then do the same on the near side. Lift the centre quill and trim it out, snip the tips off the two handles, and preen the wings into shape so they lie a little spent.
Tie a sized grizzly rooster saddle in at the base of the wings.
Spin a small pinch of dubbing and build a short thorax to seat the hackle on, ending with bare thread just behind the hook eye.
Wrap the hackle over the dubbed thorax with the turns spread just slightly apart rather than butted together, tie it off, trim the stub and whip finish.
Trim the hackle flush along the bottom so the fly sits lower and the curved hook lets the butt end hang below the surface.