you compare the circumference of the underbust to the circumference around the biggest part of the bust (your cup size).
you measure around underneath the breasts and add 5 inches (in the united states). If it measures 29 inches, then she would buy a 34 inche bra. etc.
now measure the biggest area around the bust. if this number is one inch more than the number of the underbust (i.e., 35 vs 34 inches) then she is a C.
two inch difference = B
three inch difference = C
four inch difference = D
five inch difference = DD
and so forth. there are companies that specialize in maing customized fit brassieres (sizes like Q or HHH)
i use angst