Next we’re going to make a selection of the Button by holding the Command (PC: Ctrl) key and clicking on the Button layer’s thumbnail in the Layers palette. With the selection made, create a new layer called Button Sides and click the Add Layer Mask icon at the bottom of the Layers palette to create a Layer Mask on the new layer using the selection.
We’re going to draw in the right and left sides of the button now. Switch to the Brush tool by pressing the B key and choose a round soft edged brush that’s around 8 pixels in diameter, make sure that the hardness is set to 0%. Set the foreground color to #4A4B4B and then starting at the top left corner click once then holding down the Shift key click a second time at an angle into the button where the black gradient meets the gray, then while shill holding Shift click on the lower left corner. This should create a nice angled edge as shown below. Do the same to the right hand side and fill in any areas that you may have missed at the edges.