Hold tape measure parallel to the floor under your arms at the fullest part of your chest.
Measure around the base of the neck.
Keeping the loose enough for a finger space, measure around you natural waistline. (This is usually where your waist is the smallest)
Bend your elbow and place your hand on your hip. Measure starting from the center back of your neck, across your shoulder to your elbow, and down your wrist.