Mount a 3XL nymph hook and wrap the shank with two layers of heavy lead wire, the second layer shorter than the first so the underbody tapers rather than steps.
Start black thread and bind the lead down hard, tapering the thread over both ends.
Tie in six crow or raven biots at the bend, splayed into a short V.
Catch in copper wire, then tie in thick black wool yarn.
Wrap the yarn forward as a fat, even body to the 65% mark and tie it off.
Spiral the copper wire forward over the body in 5-6 turns and helicopter off the excess.
Tie in one grizzly hackle and two or three strands of white ostrich herl together at the front of the body.
Wrap the grizzly and the ostrich forward together as a single collar over the thorax area, two turns only, so the white herl reads as gills breathing through the hackle. Wrapping them together rather than separately is the whole idea of the fly.