Mount hook, lay a smooth thread base to the bend. On small hooks, the base is half the fly.
Tie in 6-8 dun hackle fibers at the bend, one shank length. Sparse.
Dub the thinnest possible olive noodle and wrap forward to the 65% mark. On a size 20, the dubbing should be barely visible on the thread. Baetis are slim insects.
Tie in two matched dark dun hackle tips upright and divided, or a small CDC puff for the emerger look.
Wrap a dun hackle 2-3 turns behind the wing, 2-3 in front. Fibers about one and a half hook gaps.
Tiny head, whip finish.
Substitutions
Body instead of Olive dubbing: BWO Superfine dubbing — Standard match; Olive-brown rabbit dubbing — Buggier texture; Stripped peacock quill — Segmented body, beautiful but fragile. Coat with UV resin; Olive thread body only — Minimalist, great for small sizes
Wing instead of Dark dun hackle tips or CDC: CDC puff — Better floatation, very natural; Dark dun poly yarn — Easier to see than CDC; Medallion sheeting (cut to shape) — Realistic wing, more work
Tail instead of Dun hackle fibers: Coq de Leon fibers — Stiffer, better flotation; Micro Fibbets — Synthetic, very consistent split; Dark dun hackle fibers — Slightly darker, fine
Other versions
2 more in the app: Gregg Dressing (1940), Barbless.