What you need
A soft tailoring tape marked in centimetres, a mirror, a notebook, and someone to help with the back measurements if you can. Wear a fitted shirt or vest, not a jumper, and the undergarments you would wear with the finished cloth. Stand on a hard floor.
Write every number in centimetres, to the nearest half centimetre. Do not convert from inches in your head. If your tape only shows inches, convert once on paper (inches × 2.54) and send centimetres only.