Arrange the cornice pieces on the floor to see how they will need to fit on the inside and outside corners of the walls.
Hold a section of cornice up on the wall, where you want to start, and make a mark on the wall with the pencil.
Place a level on the mark. Draw a line around the perimeter of the wall to use as a reference line for installing the cornice.
Place the end of one piece of cornice onto the miter saw. Adjust the miter saw to a 45-degree angle and cut the end to create an inside corner.
Set the first piece of cornice in place on the wall against an inside corner. Secure the cornice to the wall with the nail gun. Place a nail every six inches along the cornice.
Install the next section of cornice and attach it with the nail gun. Repeat this step until you reach the opposite corner. Measure the last piece with the tape measure and cut a 45-degree corner into the opposite end.
Cut a 45-degree corner into another piece of cornice. Insert the 45-degree end of the cornice into a corner so that the cut ends of the cornice meet. Secure the piece of cornice with the nail gun.
Repeat steps 4 though 7 to cut and install the corner cornice pieces to complete the installation.