Create a new layer called Shadow Highlight directly below the Button Sides layer we just worked on. We’re now going to create an area where we can paint in a highlight and shadow without effecting the other areas.
Grab the Custom Shape tool once again by pressing the U key, select the Rounded Rectangle with a corner radius of 4 pixels but this time make sure the Paths option is selected. Click and drag a path onto the stage in the center of the button from one edge of our newly created Button Sides to the other. Press Command-T (PC: Ctrl-T) to Free Transform the path then Command-Click (PC: Right-Click) on the path and choose Perspective from the menu. Grab one of the top corners and drag it outward to expand the top of the shape until the sides line up with the Button Sides as shown below then press Return (PC: Enter) to commit the transformation.