Wrap measuring tape around your waist, snug, but not tight - this is your waist measurement.
Wrap measuring tape around your hip, snug, but not tight - this is your hip measurement.
If you fall between sizes consider the next largest size. Panties should lay smooth against the skin, never digging or pinching.
To find your natural waist, bend at the sides and see where your body indents. That is your natural waist.
The fullest part of your hips is usually about 8" below your natural waist (this may fluctuate from 6" to 9" depending on your height).