We measure each item "relaxed" on a flat surface, not stretched on a person's body. It is helpful to consider the fabric of the item, as very stretchy fabric may give an inch or more.
Bust: Measure the fullest part of your bust and make sure the measuring tape fits comfortably.
Waist: Make sure the measuring tape fits comfortably around the narrowest part of your waist.
Hips: Place the measuring tape below your waist and above your thigh. Make sure the tape fits comfortably around your body.