To start with, you choose two pictures you want to combine.
Then you press Apple+A, Apple+C and then select the other picture and press Apple+V.
You then find your brush tool select black paint and start painting in the places you want the background to show through.
You may have to press Add Layer Mask at the bottom of your toolbar to make it work!
And start painting! You can make really interesting and cool photos! Especially of series, like this one. Enjoy!