1. 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 (a dress maker's soft tape measure for preference) goes under your arms.
Keep the measuring tape snug but not too tight. If you do it too tight (if your bust is popping out over the tape) then you'll get the wrong measurements and your dress won't fit properly.
2. 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.
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.
3. 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. Again you'll want to keep the measuring tape somewhat loose, so that your dress size doesn't end up being too small.