Step 1
Measure the height of the window trim, the height of the ceiling, the width of the window trim. In general, the cornice should sit 4 inches above and to each side of the outside of the window trim. Some cornices will mount right below the ceiling in 8-foot ceiling rooms. Transfer your measurements to graph paper with 1-foot in the room equal to 1/2-inch on the graph paper.
Step 2
Draw the window and the trim. Mark the ceiling and the floor. Lightly draw lines to indicate draperies. Sketch a cornice. On a standard 8-foot ceiling room with standard windows the cornice may be 8 inches tall and 4 to 6 inches deep. By sketching these dimensions onto a scale model of your window you should be able to see if it looks good. If the cornice is too tall it may make the drape look squashed.
Step 3
Determine if your fabric of choice is wide enough to span the cornice. If your fabric has a particular pattern you may want to piece the fabric (sew it together at specific spots) to feature some design on the fabric. Each time you piece fabric you add to the amount you will need to cover the cornice.
Step 4
Determine liner fabric height by measuring and adding twice the cornice height plus twice the cornice depth, plus 4 inches. The liner width is the cornice width, plus four times the cornice height plus 4 inches. The liner should be a white fabric that covers the entire cornice board so that when you look up into the underside of the cornice you see clean white fabric.
Step 5
Calculate finished fabric height by first factoring any pattern returns. Measure cornice height, plus depth, plus six inches. Measure cornice width, plus twice height, plus six inches. (Double your width yardage for each pattern return.)