Take some sandpaper or Scotchbrite, and scuff the shiny surface of the flippy-flap as well as the loose front of the scalloped corner, top and bottom. Apply contact cement to the scalloped flap, the flippy-flap, the front of the top material, the back of the header lip and the bottom of the header up to the line of holes. If you need additional coats to get a mostly-shiny glue surface, do so. Let it dry until it is barely tacky.
If you leave the last inch of the flippy-flap unglued, it gives you something to hold on to while doing the gluing. Once the glue has tacked up and before you press it into place, you can trim off that last bit of unglued strip. It doesn’t really need to be as long as it is.