Measure correctly
Bust
Measure your bust circumference at the fullest part of your bust. So, the widest part. Please measure with a bra on, unless you don't intend to wear one under your dress.
Waist circumference
The waist circumference is the narrowest point below the bust, usually a few cm above the navel.
Hip circumference
The hip measurement is the widest measurement below the waist, usually over the buttocks. The hip circumference should not be confused with the waist circumference, which is where normal pants sit, for example.
Measurements
Bust, waist and hips are the most important circumferences to take for a standard size. Simply have them measured and compare with the size chart.
Measure
Stand up straight, look forward, and let your arms hang loosely. Therefore, it is best to have someone else measure you.