What Lydia did
The actual measurement around the fullest part of the bust, not your bra size. For bust size measure around your chest at the fullest point not wearing a padded bra. Make sure to go under your arms.
Measure around the smallest part of your waist with one finger between your body and the measuring tape.
Do this one in front of the mirror - measure around the widest point of your hips (front view) and be sure to include your bum. Stand with your feet and legs together and try to keep the measuring tape level to the ground while it circles your body.