Use a steel tape measure
Measure each window separately
Decide whether you want 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 longest height
Measure the window depth to ensure enough space for inside mounting
For outside mount, measure the width you want the blinds to cover beyond the window frame
For outside mount, measure the height you want the blinds to cover beyond the window frame
Measure in three places for width and height to account for uneven windows
Round measurements only if the manufacturer instructs you to do so
Write width and height separately and label each window clearly
Double-check all measurements before ordering