Cut out those parts you will glue down, and don't feel shy about using a ruler on the longer sides if your hand isn't very steady. Cut the side wall pieces long enough to cover the corner of the box. Do the same with the top carved molding piece. Hold the scissors at a steep angle so that the edge you cut is thinner than the thickness of the paper. The paper is this angle ____ , but the scissors are this angle (/) or steeper. You can make an impression on where to cut by lining up the edge at the back, and creasing the paper on the corner itself. You really want those corners to meet. I use a bone folder. Dry fit the moldings, and cut down the side pieces now.