A picture rail is merely a specialised type of moulding that you install as you would any other moulding. The difference is in the rounded top edge, which is slightly offset from the wall. This offset allows a hooked hanger -- usually "S" shaped -- to fit over and slide along it. Hang pictures from the rail by attaching cord, ribbon, wire or chain to the backs of picture frames, then suspending the line from the special hooks.
Establish the desired height of the picture rail. Measure from the floor or ceiling the same distance at two widely spaced points, using a measuring tape. Draw a straight line between the points with a level to act as your guideline.
Find a stud in the wall along the line you drew, using one of three methods -- a stud finder, tapping on the wall or hammering a small finish nail repeatedly into the wall until you hit a stud. Slide the stud finder across the wall until it beeps or lights up, indicating the presence of a stud. Alternatively, tap lightly along the line and listen for a hollow sound -- indicating space behind the wall -- changing to a solid sound -- indicating the presence of a stud.
Measure 40 cm (16 inches) from the first stud. Test again for a second stud using the methods described in Step 2. Most homes have framing with studs at 40 cm (16 inches) intervals from the centre of one stud to the centre of the next, though many older homes may have 60 cm (24 inch) or other spacing. Continue testing until a spacing pattern develops.
Mark all the studs across the wall, using the spacing pattern you have established, until you have covered the length of your picture rail.
Find the centre point of the picture rail with a measuring tape. Mark the moulding at the centre point.
Hold the rail horizontally against the wall between the first and last stud marks. Line the centre point of the picture rail with the marked stud closest to it. Hold the rail at that spot while you mark off the other stud positions on the rail. Make sure all the marks on the rail correspond to their respective stud locations.
Lay the picture rail on a flat work surface. Drill holes from front to back at the marked points, using your drill and a drill bit that is the size of your casing nail diameter.
Slide a casing nail into one of the end holes; make sure the nail is long enough to go through the picture rail, the wall covering and securely into the wall stud. Position the rail against the wall again, lining the other holes up with their corresponding studs at the marks.
Tap one nail into place with your hammer without sinking it completely, to allow for adjustment if necessary.
Swing the other end of the rail up into place along the line you drew. Hammer a nail through the last hole at the other end of the rail.
Place the spirit level along the picture rail to make sure it is level. Adjust one end up or down as needed, removing the nail and reinstalling it.
Hammer the remaining casing nails into their respective holes.
Countersink the nail heads slightly, using a nail punch and hammer. You also can position another nail over the head and tap with a hammer to push the lower nail head just below the surface of the picture rail.
Fill the nail holes with a bit of wood putty, applied with your finger, or a wax furniture stick. Let the material dry, then smooth the surface with fine-grit sandpaper. Paint or stain the picture rail as desired.
If you are hanging mirrors or other heavy objects, attach the picture rail with screws instead of nails for extra strength.