For best results, take you measurements over your undergarments, and use a fabric tape measure. You can also use a string and line it up to a yardstick.
Measure under the arms, by placing a measuring tape around the fullest part of your bust.
Measure with feet together, run the measuring tape around the fullest part of your hips/seat.
Using a pant that fits you well, measure from the crotch seam to the hem.
Measure 2 to 3 inches below your natural waist. Keep one finger between the tape and your body.
Measure around your waist at narrowest point (usually about your navel). Keep one finger between the tape and your body.