Use a steel tape measure
Decide whether you want an inside mount or outside mount
For inside mount, measure the exact width at the top, middle, and bottom of the window opening
Record the narrowest width
For inside mount, measure the exact height at the left, middle, and right of the window opening
Record the tallest height
Measure the window depth to ensure there is enough space for the blind hardware
For outside mount, measure the width you want the blind to cover, including any overlap
For outside mount, measure the height you want the blind to cover, including any overlap
Measure each window separately
Round measurements only if the manufacturer instructs you to do so
Write width before height
Double-check all measurements before ordering