This technique is really great, easy to make by any tiers. Even beginners, with a little bit of practice will be able to master it.
I love to fish with small cdc flies especially on slow running waters like river San from Poland. There is a lot of fishing pressure, ginger clear water and educated fish. You need good flies and good casting skills. For beginners I recommend a trip there of 3 days to understand and to discover the beauty of dry fly fishing.
There, often fish are eating only small flies like those on size #18-22. making a spent fly so small is difficult because there are lot of steps to tie a balanced fly. Then the profile should be very delicate and close to the profile provided by natural insects. It is not easy to make such flies but with proper materials a good tier can do that.
Materials needed for small spent flies
Hooks: You need good hooks with thin wire like those from Tiemco, Maruto or Varivas. The rest of them are quite bulky for such flies.
Threads: the thinnest possible: UNY 17/0, Veevus 30DEN GSP area really good for that. I recommend the the white colors because white are neutral. Then you can add dubbing, or quill or very thin threads to make the bodies.
Body: I prefer only threads, thin thread in different colors. Dubbing is not I like so much because is difficult to control a good water print with dubbing bodies. Rarely I use Kapok especially when is about dyed spent flies when the flies bodies are decomposing in the water film and the water print should be a little bit fuzzy. On size #18-16 I use Peacock quill but on small sizes I prefer tying thread in very light colors ( because the color of hook shanks will add a darker shade)
Tails: Coq de Leon – nothing else
Wings: Only CDC, I mix white with gray and dyed yellow or dyed olive. I use CDc scraps, what remains from other flies.
Tying tips: Always build a thread base when you start tying. To avoid material slipping. For example cdc barbs, coq de leon fibers and even silky thread if you use such material.
Another tip: use the thinnest thread possible to avoid bulky look. Too many turns made when the materials are added will finish your flies to look like a floating log. Keep always in mind that a natural fly is thin and delicate and super fragile. catch one and put it near your fly and you will understand why you will see fish refusing your flies so often.
I presented he the technique not a pattern. You can choose to build your own flies like you want. I used a size #16 hook for better illustration and better view.
Here is a#18 fly made with yellow Kapok dubbing.