"Pressed cotton" is alluding to the type of fabric her clothing is made of. So here "pressed cotton" could be interchangeable with the phrase "cotton fabric". the phrase "Slowly opened the pressed cotton" is a pretty way of saying "she slowly opened her clothing (which is made of cotton). "bottom edges" is referring to the bottom part of the elastic waist band of her clothing. So the phrase "Pulled out the bottom edges from her waistband" means that she lifted her waistband out from the bottom part of it". Think of the mother pulling out her pants away from her skin to show the person her scar. :] I hope that helped!

This is figurative language describing the blouse. The "bottom edges" are the bottom part of the blouse which had been tucked into her pants or skirt. "Pressed cotton" implies that the blouse is made of a cotton fabric and has been pressed (that is: the wrinkles have been smoothed using a hot iron). "Pressed cotton", in this instance, might be considered a synecdoche. http://en.wikipedia.org/wiki/Synecdoche