Glue gun. I've made probably a dozen paper masks in this style, patterns purchased from Etsy shops and printed on heavy bonded paper.

I like hot glue for this type of work because it gives you a _little_ bit of leeway to make mistakes while it's still warm, so you can slide the pieces around to get them just so. Then it cools and hardens quickly so you don't have to wait a long time before moving on to the next piece.