Step 1: Measuring

Decide on the height of your rails. A dado rail should be fitted between 1 and 1.2m (1.1 and 1.3yd) from the floor. A Victorian or older house with high ceilings should have the picture rail at a height somewhere between 300 and 500mm (12 and 20in) below the ceiling. 1930s-to-present homes have lower ceilings and the picture rail is usually level with the top of the door frames.
Use a tape measure and mark the wall at the correct height. Make marks every 1.5m (1.6yd) around the room. Work from the ceiling down for picture rail and from the floor up for dado rail.
Rest a spirit level on top of a piece of the rail and use this to join up the points you have marked. You may need to slightly adjust the line if your floor or ceiling isn't level.

Step 2: Screw holes

Make screw holes at 1m intervals along the centre of the rail with a wood bit. Countersink the holes so that you can cover the screw heads with filler.

Tip: Pre-packed dado and picture rails sold in DIY stores come with hidden plastic fixings that don't need screw holes in the wood.