If you follow the diagrammed steps literally, then the open sink of #185 is pretty hard to do (almost impossible, to me), because of the way how the two layers are entangled.
However, the sink is only needed to get spread-open in #189. So you can pre-crease the folds for the sink in #185 as good / as far as possible (using tweezers may help), but
don't actually do the open sink. Instead, in #189 just do a squash fold of the flap in question, beginning at the inner tip of the wing, slowly working your way down the flap ... until it looks like in #190.
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After my first model got halfway destroyed during fiddling with this step, I found there is a much, much easier way to do it:
Do step 185 not at step 185, but do it after step 47.
The two flaps in the wing are created in #42 (squash) + #43 (close layers). Proceed with 44/45/46 (you need to have done the long pleat of the inner layer in the model). Then, before going from 48 to 49 (turn around), you open the wing at the side again, do the open sink on the 2nd of the two inner flaps created in 42/43, spread the sink open.
This does not affect the later folding sequence in any way, it's just that the difficulty of 185/189 don't exist anymore.
Photos of the steps to do between 47 & 48:
1) side layer (wing) opened - two flaps on top of each other:
2) bring top flap to the left, crease 2nd flap for sinking:
3) open sink in progress
4) spread the open sink
5) Finished. Close all layers, then proceed with step 48.
Repeat with the other wing, of course.
Just be careful that you don't accidentially do the sink on the wrong flap!
Reminder: of the two inner flaps, one is oriented towards the head part (with the grid of square folds), the other is oriented towards the tail part (without those squares). The sink has to be done on the flap that is nearer to the tail.
) It's some sort of tissue paper, with a smooth, waxy texture.
(its not near perfect or anything but its decent)
I love that paper but I'd have that it was wayyyy too thick.
