When measuring, always use a steel tape and measure each window separately , even if they look identical. Measure twice !
Always line your curtains, they ensure they hang properly and protect from dust and sunlight.
Give your curtains an inexpensive face lift by adding a contrast border, fringe or bead.
Make curtains fuller with interlining, a soft fabric layer stitched between lining and main fabric, which insulates against noise, light, heat and cold.
...and finally, have your curtains custom made, they look beautiful, hang perfectly and last for decades.