Or How to Maximize the Room Darkening Effect of Curtains
I once read a scathing review of room darkening curtains. The reviewer complained that the "useless" curtains let light in along the top of the window. The reviewer's blame was misplaced. She should have focused not on the curtains, but on the way they were hung. If you hang curtains in the usual way on typical curtain rods, light will come in along the top. But you can hang curtains in a way that eliminates almost all light leakage.
First, hang a curtain panel on a tension rod inside the window frame. Place the tension rod as high as possible in the frame. The curtain header (the fabric above the rod pocket) should fold forward against the top of the window frame.
Pull the curtain fabric snug against the sides of the window frame. The rod should be completely covered.
Next, hang panels on a rod outside the window frame. Keep in mind that the further the rod extends out from the wall, the more light will come in. I like to use a cafe rod (the gold rod in the photo below) because it sits closer to the wall than a typical rod (such as the black rod in the photo).
To reduce light coming in along the sides, install the rod so that it's at least a few inches wider than the window opening.
With a cafe rod, you can usually pull the curtain fabric right over the bracket and the end of the rod, further blocking light.
With two layers of curtains hung in this way, you will have almost no light leaking in.