Measure under your arms around the fullest part of your chest, keeping the tape parallel to the floor.
Measure around your natural waistline. Keep the tape comfortably loose.
With your feet together, measure around the fullest part of your hips.
Lay a pair of pants that fit you well flat and measure the inside seam from crotch to hem.
It is best to have someone take the measurements for you, measuring over undergarments only.