Measure

Use a steel tape measure for all measurements.
Make all measurements to the nearest 1/8 inch.

For inside mounts (refer to illustration A), measure
the width of the window at the top, the middle and
at the bottom, then measure the height of the window
on both the left and right sides. Measure the opening
exactly, deductions for proper clearance will be made
by the manufacturer.

For outside mounts (refer to illustrations A and B),
measure as you would for an inside mount, then add
the appropriate inches to allow the blind to overlap
the exact amount you want.