A BWO emerger imitates the in-between stage of a Blue-Winged Olive hatch, when the nymph has reached the surface and is struggling out of its shuck to become an adult. That half-out, half-stuck bug can't escape, so trout eat emergers more confidently than the duns riding on top. Most patterns hang in or just under the film, often with a trailing shuck of dubbing or Antron at the back.
Tie or buy it in olive, sizes 18-22. Size 18 is the default early in a hatch; drop to 20 or 22 as the bugs get smaller or the fish get picky.
Fish it during a BWO emergence - overcast, drizzly afternoons in the 44-58F water-temp range, in clear to lightly stained water on low or normal flows. Watch for soft, gentle rises where you see a snout but no splash; those fish are taking emergers, not duns. Cast up and across and dead-drift it with no drag. If you can't track it in the film, hang it 8-12 inches off the back of a visible dry as a dropper.