You need to measure your natural waist size. You can measure your natural waist size one inch above your belly button.
Once measured, always choose a corset 4 inches smaller than your natural waist to achieve the 'hourglass' effect. If you have a large stomach go up a size. If your stomach is relatively flat and you only want to achieve a smaller stomach/waistline go down a size.
Please carefully read each shaper/waist trainer description