[header] How to make a cornice [title] Measure your window. [step] The cornice goes above the window and has to leave space for any curtains or blinds you'll have under it. Measure the window's width then add a few inches to compensate for that. 
Add another one or two inches for the thickness of the boards you'll use to make the cornice. [substeps] A good rule of thumb is to add six inches onto your initial width measurement.