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How to hang blackout curtains
Measure the length of your window frame.
Hook a tape measure on a top corner of your window frame and pull it down to the bottom edge of the frame. Then do the same on the other side and in the middle, in case your window is not perfectly square.
If you already have a curtain rod, you only need to measure from the curtain rod to where you want the curtains to end. If you do find that your window is a different length on 1 side, keep the longest measurement.