This is a good fly for early summer. Damsel flies hatch throughout the early part of the summer, making them a particularly good nymph for the season. And their size makes them a good fly for many species, in both cold and warm water.
Hook: Mustad 3665A 6XL wet fly hook size 8-12
Thread: Black monocord 3/0
Tail: Ginger goose biots (3)
Body: Olive dubbing (just about any wet fly dubbing will do)
Wing case: Wild turkey tail
Throat: Wet hackle, dyed olive brown
Eye: Melted monofilament
There is nothing mysterious about tying this nymph. It's a little longer than most with a relatively longer thorax, and uses three biots for the tail and monofilament eyes. But other than that, it's a standard nymph and should be tied as such. Foundation, tail, dubbing, rear wing case, dubbing, fore wing case, throat, eyes, dubbing, wrap knot. Cement the wrap knot and wing case.
In still water, this fly is best fished with a slow swimming motion, punctuated with long pauses. It's best fished around structure that breaks the surface, logs, stumps, reeds, anything that shows through to the surface. Lacking that, around submerged timber or close to a steep shoreline dropoff or vertical shoreline bankment. It also is effective around rocks.
In moving water, fish it as you would any nymph, keeping plenty of slack in the line to avoid drag.
The best time to use it is when you see damsel flies around the shoreline, clinging to vegetation and other structure. As adults, they prefer remaining close to water, so if they are present in the water, you will generally know it fairly quickly.