Measure all your windows, as the sizes may vary. Then measure 3 places horizontally & 3 places vertically, in case your windows aren't 100% square.
One of the first things to decide is where your blind/curtain will sit - the inside or outside the window frame (they provide different dimensions).
You have to determine the rod thickness, bracket installation, rod length, amount of top treatments and finally the number of pinch pleat panels.
Horizontal & vertical blinds, with inside or outside mounts, have different measuring processes. Shutters & roller shades also have variables to consider.