How do I set up pure:unmask?

Load pure:unmask on the channel of the background track (the track that will make space for the prioritized track).

Route a sidechain input from the prioritized channel (the track that should take center stage). Please note that how to route a sidechain differs depending on your DAW. This article might help.

Select the signal type of the sidechain signal as the profile for pure:unmask. The plug-in can then focus the unmasking on frequency regions that are most relevant for this signal type.