Start the Black thread and take it back to the bend.
From one Grizzly feather, pull 10 or 12 of the longer fibers from the fatter end and tie them in at the bend as a fairly long tail. Snip or bury any curled butts.
Strip a couple of inches of barbs off the same feather to expose quill. Tie the quill in a few millimeters forward of the bend, thinner end to the rear, and bind it back to the tail. Leave the thread short of the eye with room for four or five hackle turns.
Grip the quill with hackle pliers and wrap it forward to the thread. Do not worry about small gaps; the resin fills them.
Tie off the quill with two or three tight wraps, pulling the thread up hard so the quill cannot slip back, take a few more wraps, snip the excess and squash any nub flat.
Put the smallest drop of UV resin on the quill body, spread it around with a bodkin while turning the fly, and cure about 15 seconds.
Take the remaining thinner end of the same Grizzly feather, take one wrap back so it catches in at the front of the body, and bind the stem along the shank. Trim the stub.
Wrap three turns of hackle for a sparse mosquito, or five for a fuller mayfly. Secure with three wraps and snip.
Check that the eye is clear, pull the hackle back, make room, and whip finish.