Wrap .025 Lead Wire over most of the shank, leaving a little bare hook at the bend and behind the eye. When you reach the front, wrap the lead back over itself to build a big, heavy hump: a stonefly's thorax is bulky compared with the rest of its body, and this sets the shape.
Start the Tan UTC 70 thread and wrap through the lead, building a thread dam at the front and back so the lead is wedged and cannot slide. This does not have to be pretty; it all gets covered.
Take the thread to the rear and dub a small round ball of Golden Stone SLF Dubbing (use a shaggy, long-fibred dubbing; stoneflies are leggy bugs). The ball is the wedge that splits the tails.
Tie one Tan Goose Biot on each side of the shank, natural curve splaying outward from the body, about a quarter of the body length. Wrap the thread back to the dubbing ball so it forces the tails apart, then advance and trim the butts.
Tie the D-Rib in along the side of the fly, stretching it as you bind back all the way to the tail. Let it hang.
Spin the Golden Stone SLF Dubbing in a dubbing loop, staggering it thin at the top and thicker down the loop so the body tapers, and wrap forward. A large stonefly usually needs a second, shorter loop. Stop just short of the thorax hump.
Spiral the D-Rib forward in open turns to segment the body and bind the shaggy dubbing down; tie off.
Cut three sections of Mottled Turkey Wing graded small, medium, large, treat them with brush-on super glue (or Flexament), and snip a small V notch in the tip of each. Tie in the smallest first, centred on top with the notched end pointing back; bite down hard with the thread and trim the butts.
Dub a short segment directly on the thread (no loop here), thicker than the body, then tie in the medium wing pad so it slightly overlaps the first without covering it. Repeat with another dubbing segment and the largest pad, which should engulf the top half of the fly. Leave room at the front for antennae and the head.
Tie in two Tan Goose Biots at the front as antennae, curving away from the hook just as the tails do.
Dub a very sparse head, just enough to cover the thread (dubbing wax tames a shaggy dubbing), wrap to the eye and whip finish; whip finish under the biots if you want them standing up.
Brush out the underside of the thorax only to suggest legs. Trim any long fibres at the rear; the abdomen should not be too shaggy.