Portraiture -

I am going to being going through the process of removing blemishes on somebody's face.
I opened my image, and the first thing I did was converted it to CMYK colour.

This is under the image tab.

Then, you go to channels and select the yellow channel.

 Once this has been done, we select all then copy it, or cmd+A, cmd+C

Now this is selected, we can go back to the layers, and make sure that the background is converted into a layer before pasting the yellow channel (cmd+V)

You should then have two layers; one with a normal colour, and the other with the yellow channel.