The left control: rotates the stencil.
The right control: shift it up or down relative to the other controls to determine the stacking order.
According to Lan Ho, there are more than 100 puzzles. That's probably too many by about 50. Why? Because these get really hard very quickly. I was stuck on #6 before I realized that the right circles could move up and down. #7 is totally different from #6, but still quite hard. And that's where I stopped! At least for now.
Layer Merge requires concentration!
Other Stencil Puzzles:
Factory Balls Excellent
Art of Perception: Ravioli Highly polished.
Motif Mask Kaleidoscope Puzzle