Use your exact measurements when selecting your size.
Measure over your undergarments, and keep the measuring tape firm, but not tight.
Measure around the fullest part of your bust and across the shoulder blades.
Measure around the natural waistline, keeping the tape comfortably loose.
Measure around the fullest part of the seat, approximately 22cm or 9" down from the waist.
Please also check the product description of the garment you are wishing to purchase for notes on whether the style is a smaller or roomier fit than usual.