Add a small bead at the eye if you want it to ride a touch deeper, then tie in a short tail of mottled partridge or wood-duck fibers about half a shank long.
Tie in the wire rib, then dub a slim, tapered grey abdomen over the back two-thirds, callibaetis nymphs are slender, so keep it thin.
Counter-wrap the wire rib forward in 4-5 turns to segment the body.
Tie in a pheasant-tail wing case pointing back, then dub a slightly fuller grey thorax.
Add a sparse partridge beard for legs if desired, then pull the wing case forward over the thorax and tie it down.
Whip finish and pick out the thorax so the fly looks buggy and lifelike.
Keep the whole profile slim and tapered, callibaetis nymphs are agile swimmers, not fat crawlers.