Measure under your arms around the fullest part of your bust/chest, keeping the tape parallel to the floor.
Measure around your natural waistline. Keep the tape comfortably loose.
Stand with your heels together and measure around the fullest part of your hips. (Approximately 8" from your waist if you're average height, and 7" if you're petite.)
It is best to have someone take the measurements for you, measuring over undergarments only.