1. Open two new photograph documents. They should be the same Image Mode (such as: both RGB color) and the same resolution.
2. Use the Move tool and place it on top of one of the photos and click down, hold and drag until you reach the next photo. (If you hold down the Shift key it will center the positioning of the photo).
3. The photo that was moved is placed automatically on top of the other photo document and on a new layer. You can position the photo where you want it.
4. Go to the Layers menu and select Add Layer Mask.
5. In the Toolbox click on the default colors button to make the foreground black and background white.
6. Select a medium size Paintbrush (set it to 100% and Normal Mode) and sweep through the parts of the top photo (layer) to erase it. To replace image areas change the foreground color to white and paint through it. Remember to paint only on the Layer Mask.
7. Experiment with changing the brush size and opacity. Use can also use this technique by selecting an area ( with one of the Selection tools) in one photo document and then continue from Step Number Two.