The Power of Gray - The Photoshop: Professional Retouching and Compositing Tips, Tricks, and Techniques - page 82

Figure 2.7
5. With the scene layer active, click the layer mask icon at the bottom of the Layers
panel. Then with a normal medium soft-edged brush and a black foreground color,
paint over areas of the image where the background is showing through ( Figure
2.8 ) .