Oh, the classic Christmas lights in the window! That's a classic decor idea that can really help set the mood for the holiday season. You'll want to start by measuring your window to determine the length of the light string you need. Then, plan out where you want the lights to go, making sure to leave enough slack to allow for adjustment. Once you have that figured out, use a ladder or step stool to reach the top of the window and begin hanging the lights. Make sure to secure them tightly so they don't slip or come loose.