Use L brackets. They come in several different sizes and the size of your cornice board will determine the size of the L bracket. Measure the cornice board on the inside and attach the L brackets to the wall so the cornice board will set on the brackets. If the cornice board is extra long you may want to add more L brackets in the middle. Place the cornice board on the L brackets and screw it to the bracket. The L brackets can be found at Lowes, Home Depot, Walmart, etc.