Bead the caddis hook with a 3.5mm black nickel tungsten and start yellow Sumo 50 denier thread.
Tie in the gold tinsel rib at the bend, then dub a slim olive Belgian rabbit body forward to the 70% mark. Getting the dubbing quantity right is the entire lesson of this fly. Too much and you have a bulky thing that looks nothing like a rhyacophila larva, too little and the fish leave it alone.
Rib forward with gold tinsel in five or six open turns and tie off.
Dub a contrasting salt-and-pepper squirrel thorax behind the bead, keeping it deliberately spikier than the body.
Pick the thorax out and whip finish behind the bead.
Fish it dead drift along the bottom. Size 12 is his default; go to 10 in bigger water and 14 in smaller.