How to measure yourself correctly to choose the right underwear?

Start with the circumference of the chest
With a tape measure, measure the circumference of your chest just below your chest. Make sure to keep it firm and not loose at all.
Measure your chest circumference
Put on the simplest and most basic bra that you have in your wardrobe and pass the tape measure around the circumference of the chest, but without pressing it.

To find the exact number, you will need to subtract the chest circumference from the chest circumference. Depending on the resulting difference, you will choose the size based on the table.
For example, with chest circumference: 70 cm and chest circumference 90 cm, we have, 90-70=20 which means your cup is D, and the bra size you should buy is 70D.