REAR HOOK: mount a short-shank hook (or shank) and start thread; tie in a full marabou tail one and a half shank-lengths long at the bend, add a few strands of flash each side, then tie in a schlappen or webby saddle hackle by the tip.
Dub or build a short body forward, palmer the hackle through it in open turns, tie off and trim to make the rear section; whip finish and set this section aside on the connection loop.
FRONT HOOK: mount the front hook, lash lead dumbbell eyes on top near the front third with figure-eight wraps, and join the rear section to the front shank with heavy braid or wire so it articulates.
Tie in another schlappen hackle behind the eyes and palmer a dense collar forward to the eyes; tie off. This is the moving, breathing front hackle.
Spin the first stacked clump of deer belly hair in front of the eyes: hold it on top, take two loose wraps, then pull tight and let it flare all the way around; pack it back hard against the hackle.
Spin and pack successive clumps of deer hair forward, filling the shank tightly to the eye and packing each one back before adding the next, until the head is dense.
Whip finish at the eye, then remove the fly and trim the spun hair to a blunt, flat-faced bullet head, squaring off the front so it pushes water.