The idea is to make the collage effect look as though each individual Polaroid photos were taken separately. You rotate and move the image of the woman so that they are not in perfect alignment with the background image.
Note also that the Polaroid images are flat in the above example.
Here I have added in the Drop Shadow. I made the drop shadow tight so it would appear as though the Polaroid photos are sitting on top of the background image and not hovering above them. Personal choice I guess.
Notice that even with the drop shadow they still appear flat?
Let's add a slight gradient to each Polaroid and some edge highlights to see if we can improve them and make them look more real.
Since I'm not sure about your skill level in Photoshop I have not gone into any details about how I did it. If you have questions just ask.