If you are going to install yourself, and these are final measurements, then follow the guidelines below: First, number the window openings going around the perimeter of your porch clockwise. The windows will be labeled in this order when they arrive. Second, make sure that you have a flat surface on the outside to attach the windows through the 1” window flange. If you don’t have a flat surface of at least 1” around the perimeter of the openings, then additional framing will be required. Third, record the width and height measurements of each opening by checking in multiple places and looking for the smallest number. We typically make the window units ¼” smaller in width and height from the opening to allow for clearance. When providing the sizes for self-installation, please give us the final frame “make-size” that you want us to manufacture the window frames to.