Little seamstress advice:
Heat your measuring tape in your hands.

Chest circumference:
Measure your chest circumference as strongly as possible, supple and on a soft bra if possible.

Underbust:
Take your turn under the birth of the breasts, tightening slightly the measuring tape.

Size
Take the measure in the hollow of your size: this measurement is random and moves a lot.

If in doubt, refer to the measurement of the hips.

Hips:
Measure at the fold of your legs and at the height of your buttocks.