In order to peep under Juliet's skirt you must manipulate the camera angle by moving the right thumb-stick () up/down (depending on your settings) to be as close to the ground as possible. Hold it there for a couple of seconds. Stand still and Juliet will stick her chainsaw into the ground allowing you to get a peek. It's best to do this while there are no zombies or any type of danger around. This achievement is not as exciting as it would seem as Juliet will cover herself up as you move the camera. Simply push forward until you're either in front or behind Juliet and she pulls her skirt down and it will eventually pop. This can be done at any point during gameplay.