Measure inside the opening in 3 places. Enter the smallest width and the smallest height on the order form.
Measure the width inside the opening in three places. Measure the height from the top of the sill to the inside top of the opening in 3 places. Enter the smallest width and the smallest height on the order form.
Measure the width from the outside edge of the window trim to the outside edge of the window trim. Measure the height from the top of the sill to the outside of the top trim in 3 places. Enter the largest width and the largest height on the order form.
Measure the width from the outside edge of the window trim to the outside edge of the window trim. Measure the height from edge of the bottom trim to the outside edge of the top trim in 3 places. Enter the largest width and the largest height on the order form.