For accurate measurements, it's best to take yours when you are wearing your most comfortable bra - one that has no padding and is not a minimiser.
Step 1 - Band size
Wear a non padded bra and use a soft tape measure. Hold one end of the tape underneath the front of your bust then circle the tape around your back at band level. If the measurement ends on a half centimetre/inch, round it up.
Step 2 - Bust measurement
Wear a non padded bra and use a soft tape measure. Hold the tape at nipple level across your bust. If the measurement ends on a half centimetre, round it up.
Step 3 - Cup size
Subtract your band size from your bust measurement. The resulting number equates to the letter of your cup size.