Quote - Where does the second image go? 

I've read dozens of tutorials and people keep answering, but there's no mention of where to apply the mask.

 

You first plug a Blender node into the Displacement Channel, then plug the Mask into one channel of the Blender and your Displacement Map into the other channel.  Set the parameter to 1 and plug the Mask into that also.

I can't remember offhand which the two maps go but since there are really only two options it's easy enough to work out.  When it's working you'll see an image with your displacement map in the white areas and plain black in the rest. 

Coppula eam se non posit acceptera jocularum.

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