In a new document isolate polaroid shape. Take stock photo, place on layer behind polaroid re-shape/size stock to liking.
remove any excess showing from behind polaroid(duplicate polaroid layer now to save having to re-isolate), merge polaroid and stock layer.
Add shadow effect
Repeat with each photo you with to 'polaroid'
step 3 of 4
drag each completed polaroid layer over to document with background image and resize/move as required.
step 4 of 4
Finally add text if desired and rotate to fit each image (i did it this way but putting text onto each polaroid layer and grouping as a smart object before you resize/rotate would possibly be easier as it would negate the need to fiddle about getting text to look right)