Mat is just a variable that you have to create. That the part “Drag off the [Post Process Materials] pin [from the Make Post Process Settings node] and promote it to a variable”. And Post Process is the Character Component that you lack for some reason.
Sorry, just from the way you described everything in the beginning it seemed to me that you’ve already figured out the post process part, and just wanted to know if it’s possible to apply it to only one camera.