Prep a dark brown rabbit zonker strip: trim the fur off the front/top portion, leaving about a half inch of fur at the tip. Tie it in on top of the shank at the very back of the hook so the furred tip hangs off the bend as a tail roughly one to two times the length of the whole fly.
Cut a strip of black thick foam about as wide as the hook gap and trim one end to a taper/V so the tapered section is about a shank length long. Tie the thin pointed end in near the back of the hook, leaving the foam pointing forward to be pulled over later.
Cut a pencil-diameter clump of deer hair and trim it to about an inch long. Lay it on the shank with only about a quarter inch of butt ends forward of the tie-in point, take one loose wrap then pull tight to flare and secure. Push the fibers back, lay the thread in front, and repeat, packing clump after clump forward along the shank.
For the last clump at the head, tie in a bunch with the butt ends left as long as the tips and spin it around the shank, then pull everything back and work the thread up to the eye.
Split and part the packed deer hair and snip a shallow channel along the top. Stretch the black foam forward over the top of the body, forcing the deer hair down and away from the eye. Secure with two loose wraps then pull tight, and whip finish over the front while holding stray fibers clear.
Trim the foam to leave about a quarter inch tab facing forward off the eye. Trim the deer hair flat and close to the shank on the bottom, all the way back to the tail, then taper the top and sides to a mouse-body profile. Add a drop of superglue or head cement to the head.