Measure the circumference at the fullest part of your chest and across your shoulder blades.
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.