Stand on a ladder and measure from the ceiling down to where you want the border. A general guideline is the molding should be at an 8-foot height.
Make a small mark at the height you want the molding with a pencil.
Line a laser level so the laser is on the height you have chosen. Many laser levels will project a line onto all four walls. If not, move the level around the space to follow the same line.
Apply a strip of tape around the entire room just below the laser line.
Paint the ceiling and the wall above the tape the same color. Allow it to dry and apply a second coat. After the second coat dries, you will see whether you need an extra coat.
Pull the tape off the wall when the paint is dry.
Measure the length of the walls to determine the length of molding needed.
Cut the molding to size with a circular saw.
Paint the molding a few shades darker than the wall color. Set it aside until dry.
Set the laser level on the wall again to give you a guide while installing the molding or position the molding even with the line where the wall color meets the ceiling.
Apply strips of painter's tape to the molding every few feet to keep long pieces level.
Attach the molding to the wall with finishing nails and a hammer. Hammer a nail every 6 to 8 inches.