Mount hook, tie in a poly yarn post at the 70% mark. Stand it up with a thread dam and wrap 8-10 turns up the post base.
Tie in split dun tails at the bend, 3-4 fibers per side.
Dub the thinnest olive noodle you can and wrap to the post. Baetis are slim; halve your dubbing.
Tie in a dun hackle at the post, shiny side up, and wrap 3-4 parachute turns, each below the last. Tie off under the hackle.
Whip finish. The body should sit flush in the film under the flat hackle disc.
Substitutions
Body instead of Olive dubbing: BWO Superfine — Standard match, easier to dub thin; Olive-brown dubbing — Darker variation for fall BWOs; Olive thread body — Minimalist, great for tiny sizes
Wing post instead of White or gray poly yarn: Hi-vis orange poly yarn — Much easier to track in flat water; White Aero Dry Wing — Better flotation; CDC post — More natural but less visible