Measure your existing window to determine what size window to order. First, measure the width of the inside of your window jamb at the top, middle and bottom (Image 1); then measure the height at the center and the right and left sides (Image 2). Take the smallest reading in each set and use those numbers when ordering your windows. Most suppliers will order to the closest 1/8", so give exact measurements when ordering. It's better to err on the small side than on the large side when ordering windows.
Once your storm window has arrived, test-fit it to make sure it's the right size (Image 3). It's also a good idea to wash the regular window before installing the storm window.