Question: how to cmyk color separations in photoshop?
Answer: Open a copy of your artwork in Photoshop. If the image has layers, use the menu command Layer> Flatten Image. ... 
With the Cyan file active, use the menu command Image> Mode> Duotone. ... 
To show the “build” of color, make the Cyan file active and use the menu command Image> Duplicate, naming the new file Cyan-Magenta.