Note that depending on the store, exact sizing differs.
Check online to find the store's sizing guide and compare that to your own measurements.
Measure your bust
You'll need to measure the fullest part of your bust to get the proper measurement.
Make sure that the measuring tape goes under your arms.
Measure your waist
Bend to one side (doesn't matter which) and find your waist's natural crease. At the crease, measure around your waist, making sure that the measuring tape is a bit loose.
Measure your waist
You can also find your natural waist by measuring 2 inches up from your belly button. It's usually the smallest part of your waistline.
Measure your hips
Stand with your feet together. Measure around the fullest part of your hips and rear. This is typically midway between your crotch and your belly button.
Measure your hips
Again you'll want to keep the measuring tape somewhat loose, so that your dress size doesn't end up being too small.