Your measurements will be most accurate if someone else measures you. Make sure your feet are directly under your shoulders, and let your arms hang down by your sides (see photo for reference).
Measure the circumference of your bust at the fullest part of your chest. Under your armpits, around your shoulder blades, and around the front of your chest. Please keep in mind that your bust size is not the same as your bra size.
Measure the circumference of your waist at your natural waistline. If you bend to the side, the crease that forms is your natural waistline.
Measure the circumference of your hips over your hip bones and around the fullest part of your buttocks.
While standing up straight, have a friend measure from the crotch seam to the bottom hem of your pants. If measuring by yourself, lay a pair of pants flat, and take the same measurement.