Note: Curled corner made by cutting a triangle from corner..paste the cut corner back on the workspace and then transform the corner triangle via transform horizontal and vertical directions.. add shadow, split out shadow and then using warp tool, shape the corner triangle and then do the same warping the shadow.. add a gray fill layer set to overlay over the corner triangle and paint with black and white for burn and dodging to create more dimension on curl and then add a black painted line under the straight edge of the triangle lower opacity and glaussian blur the black line for shadowing.