A new Image 600 x 600 Transparent Background. Add a new layer Label it Backside. Hide the layer label backside for the rest of the tutorial. Click on the deform tool. Add a new layer and with your selction tool draw a thin rectangle on the top and all the way across.
Flood fill with a darker wood pattern or the same one.
Copy that selection and paste on a new layer. Place below the first line then Paste as a new Section again and place this Under that line. Hide the front piece layer and Layer 1 and make sure the backside layer is closed also. Click on the deform tool and holding the control key down Move the top box in until it fits flush to the cabinet. Move the bottom box in also to your liking.