Hold the corner piece in its installation place to ensure it lies flat on the wall. Mark imperfections that prevent the corner piece from lying flat on the wall with a pencil.
Sand the imperfect areas of the wall by hand with fine-grit sandpaper. Check the fit of the corner piece periodically as drywall does not require a lot of sanding to make a reasonable amount of change in the surface. Keep sanding until the fit is within 1/8 inch flat in all areas that the corner molding comes in contact with the wall.
Wipe the wall with a microfiber cloth to remove the sanding dust.
Place the corner molding and position a pneumatic nailer so that the nail will pierce through the side of the corner molding at an angle and enter the wall within one inch of the corner, hitting a wall stud. Use 2 1/2-inch finishing nails and do not place the nails within 1/2 inch of the edge of the corner molding, or you may split the molding.
Nail the other side of the corner molding using the same method. You will probably need to nail each side twice in order to make the corner molding as sturdy as needed. When firmly connected, the molding should barely shift with medium pressure from your hand. Do not worry if slight shifting occurs as caulking will help firm up the connection with the wall.