Use a steel tape measure
Decide whether you want inside mount or outside mount
Measure the window width at the top, middle, and bottom
Record the narrowest width for inside mount
Measure the window height at the left, middle, and right
Record the longest height for inside mount
Measure the window depth if using inside mount
Check for obstructions such as handles, locks, or cranks
For outside mount, measure the full area you want covered
Add extra width and height for better light control and privacy
Measure each window separately
Round measurements to the nearest 1/8 inch
Write width first, then height
Confirm all measurements before ordering