I still would not cut the liner first. Punch two holes into the liner in the top right and left corner screw holes. Place the gasket over the wall. This type of gasket is meant to protect the wall against rust. Hold the faceplate in place with the two topscrews into your prepunched holes. With the skimmer in your left hand and a screwdriver in your other hand start the two top screws. Recheck your gasket and start two lower screw. Now proceed to install and tighten all the screws. Then cut your opening. It's pretty easy.

Dennis