Place tape measure loosely under the arms (over the shoulder blades) around the fullest part of the chest.
Measure around the natural waist keeping the tape measure slightly loose.
Measure around the fullest part of the bottom at the top of the leg about 20cm (8") below natural waistline.
Dress lengths are taken from the back neck (nape) to the hem. Skirt lengths are from under waistband to the hem. Trouser lengths use the inside leg measurement.