It is recommended that you get someone to help you in order to get the most accurate measurement.
Waist- Stand with your arms by your side and measure around the narrowest point of your waist. Most people find this is just above their belly button. Take a relaxed stance ensuring not to breathe in.
Hips- Stand with your feet approximately 6 inches apart, and measure round the fullest part of your seat. Again, with you arms by your side.
Length- The length measurement is taken directly down the front of the skirt.