Wear a non-padded bra or no bra
Measure the underbust by wrapping a tape measure snugly around the ribcage directly under the breasts
Measure the bust by wrapping the tape measure around the fullest part of the breasts, keeping it level
Record both measurements in the same units
Subtract the underbust measurement from the bust measurement
Use the difference to determine the cup size according to a bra size chart
Combine the underbust measurement with the cup size to find the bra size
Recheck the measurements for accuracy
Measure again if the body position or tape placement changes