Using layer masks in Photoshop is a non-destructive way to edit an image. You use the masks to hide or reveal parts of the image, but by using the masks you are not harming the original image.
To use layer masks in Photoshop, simply open the image you want to change (we will call this the background image) and the image you want to overlay onto it (we will call this the overlay.
Now drag the overlay image on top of the background image.
If you need to resize it hit CTRL + T to resize. You can hold down the shift key to keep the shape of the image and not distort it. Once you have it the size you want just click the checkmark button at the top to ok it.
In the layer palette choose the overlay image and click the layer mask button at the bottom. (It’s the rectangle with the circle in it.)
The mask should be white which means it is revealing everything. (White reveals and black hides or masks the image!). We want to use black so you can hit x on your keyboard to switch you from white to black.
Using a paintbrush and a black layer mask paint over your image to reveal what you want to see in the background image. (If you have difficulty seeing it, try changing your blending mode to multiply or screen and then reducing the opacity.)
If you make a mistake, hit the x and it will go back to white and you can paint over the area you want to fix.
Adjust your brush size as needed.
Once you are finished change you blending mode and opacity to suit the image. If you can see brush strokes in a particular area, you can always use the smudge tool to minimize them. If you don’t like the look obtained by using the layer mask just delete that layer and start over!