When buying curtains online, first consider what room you're using them in and how much light you need them to block. Maybe a living room just needs a thin enough curtain to dim the lights and frost the windows, while a baby's room needs thick curtains to block out light completely. Next, choose the color you want based on other furniture and decor in the room. You'll need to measure the height and width of your windows to ensure you get the right length and have plenty of coverage.