Drop the water level of your pool to 2 inches below the skimmer faceplate.
This step will require a magnetic screwdriver. Remove the screws securing the faceplate to the pool wail. A magnetic screwdriver is important in this step because you do not want to go fishing for screws later on.
After the screws are removed the faceplate and gasket should peel off pretty easily. You may need to pick a corner with your fingernail to start the process. Be careful not to scratch the liner.
To prevent liner shift place the screws removed in step 2 in their respective holes while you are preparing the new gasket for installation
With the new gasket at the ready remove the screws and set gasket in place around the mouth of the skimmer. Line up the holes of the gasket with the corresponding screw points on the skimmer face.
Place the skimmer's faceplate over the gasket and secure with screws. Run your finger around the edge of the faceplate after tightened to make sure there are no gaps and the plate is evenly tightened.