For an inside mount:
Measure the opening width at the top, center and bottom, recording the narrowest width. Always measure to the nearest 1/8", and be sure to use a steel tape measure. See tip below.
Measure the height left, right and center, recording the tallest height.
When you specify inside mount, the factory will make your blind slightly narrower than you specified to allow for operating clearance. They will deduct between 1/16" and 3/8" from the width on each side. The deductions are made so that the blind will not rub or scratch your window frame. So do not take the deduction yourself. Provide the exact window opening width.
For a flush inside mount (the blind completely recessed so that the top of the blind does not project out of the opening) check that you have enough depth.