What you are going to make out of the box will determine the kind of materials you choose for its construction. Okay now you have the box and the plan, what do you do. Cut-outs are shaped cut out of the box. I have drawn a couple of then for you to see. Wood or stenciled applique are very attractive finishes for cornice face boards.
Most cornices get upholstered after the cut outs are made.
The upholstery has taken a twist in modern times with the use of a wider range of materials. Do not wrap the batting under the bottom edge or behind the top or leg, just the surface of the face leg and top. Welting can be made or purchased ready made. The drawing for welting construction will help you gage your tab width. You staple the welting to most cornices between the folds of the tabs.