Taking my own measurements and getting a different answer every time

Commissioning something properly made for the first time and I am stuck at the very first step.

I have measured my waist four times over two evenings and got four different numbers, spread over about three centimetres. Same with the hips. I have no idea which of them to send.

How tight should the tape be? Do you breathe out or breathe normally? And is measuring yourself simply a bad idea that everyone quietly gets someone else to do?