Place an index finger under the tape to ensure a comfortable fit, and measure around the fullest part of your neck where your shirt collar would sit.
Lay the collar of a shirt flat and measure from the center of the collar button to the far end of the buttonhole.
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.
It is best to have someone take the measurements for you, measuring over undergarments only.