We're starting with a simple line art image, which only has the one "Background" layer.
In the Layers palette, make sure the Background layer is selected. (Hey, it doesn't have much choice!) If you don't have a Layers palette, use Photoshop's Window pulldown menu, and click next to Show Layers.
Switch from the Layers palette to the Channels palette. I'm usually working in RGB mode by now, so all three channels-- Red, Green, and Blue-- show as selected. Select just one channel-- I'll click on Red here.
I did that so I could use the Channel palette menu's Duplicate Channel... option. (Which won't work with all the RGB channels selected.)
Check the Invert box, and give the new channel a name; I used "Line Art Channel mask." (Original, huh?) Click the OK button.
The result looks like a photographic negative of the original linework.
Click on the RGB (master) channel, to put the view back to normal, and switch back to the Layers palette.
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