In this how-to video from Porta, Ed explains how to install a picture rail.
A picture rail not only provides another decorative element to the wall, but also gives you the option to position wall hangings anywhere on the wall without having to rely on the positions of studs.
In an older Victorian style home, picture rails should be 300-500mm from the ceiling, while in modern homes with 2400mm ceilings, the picture rail should be aligned with the top of the door architrave.
To ensure the picture rail is level, create a spacer equal to the height between the ceiling and the top of the door architrave. Use this as a guide when installing the picture rail, and secure the picture rail to the studs using nails.
Use a mitre or scribe joint for internal corners, and join any long lengths using a splice joint.