Drill a 5/32-inch pilot hole though the plastic tabs on the molding so that the screws go easily through the molding.
Find the studs in the wall with a stud finder. Mark the location of each stud with a pencil. Try to screw the molding tabs into the studs whenever possible.
Install the corner blocks. Line up the plastic block with the ceiling and wall. Screw the block to the wall through the plastic tabs. Do not sink the screw too far into the wall or it could crack the plastic tab.
Measure the distance on the wall between the blocks. Mark this distance on the plastic length of molding.
Cut the plastic molding to size with a miter or hand saw.
Apply adhesive caulk on the back of the plastic molding in an "s" pattern.
Slide the molding into place between the blocks. Use a friend to help you do this when you are working with long lengths of molding.
Screw the length of the molding in place with a 2-inch trim-head screw. Screw the molding directly to the studs in the wall.
Fill the screw holes with paintable caulk. Let the caulk dry and then paint.