Measure around the fullest part of the chest, keeping tape measure up under the arms and around the shoulder blades.
Stand up straight, measure from the base of the shirt collar down to the bottom of the pant waist band.
Measure around the fullest part of the waist / stomach or right below the bottom rib, whichever is greater. Keep one finger between tape and body.
Measure around the base of your neck, make sure the tape measure is snug, but comfortable with movement.