how do i measure a window properly?

Is this for blinds or a replacement window? The basic method is the same: measure the width in three places (top, middle, bottom) and the height in three (left, middle, right), all inside the frame, and use the smallest of each. Old frames are rarely square, so measuring once is how people end up with something that doesn't fit. Measure both diagonals too, if they differ a lot the frame's out.