How do I measure for an outside mount?

Outside mount is the forgiving one, because you control the overlap. Decide how much the covering should extend past the opening — a common guideline is at least 2–3 inches beyond the opening on each side and above — then measure the total width and height you want the blind to cover, including that overlap. There’s no factory width deduction on an outside mount; you order the finished size you measured. Make sure there’s enough flat wall or trim to land the brackets on, clear of any obstruction, and that the covering will clear window cranks or handles when lowered. Because it overlaps the frame, an outside mount blocks more light and hides an out-of-square opening — just confirm the mounting-surface and bracket details against the seller’s guide first.