Take a deep breath, let the air out through your nose and stand up straight. Stand in a comfortable, natural stance. As tempting as it is, don't suck in your tummy, -- stand naturally.
Keep your arms resting against your body as your partner uses the tape measure to start measuring around your hips. Your hips will be the widest part of the body below the shoulders, so don't let this measurement throw you. Measure with the tape measure resting evenly all the way around your body. Write down this measurement, exactly as it is measured.
Measure your waist, which will be the smallest part of the core, near your belly button. Again, have your partner keep the tape measure flat and even to get an accurate measurement. Write down this measurement exactly as it is measured.
Finish with a bust measurement, covering the back shoulder blades, under the armpits and around to the front of the fullest area of your bust. If you are uncomfortable with your partner doing this, have them hand off the tape measure to you, and read off the measurement so she can write it down.
Compare these measurements with the averages collected over time and entered into a database at websites such as WomensSizeChart.com or WmsClothing.com or the store where you're shopping to decide on the average dress size that is right for you.