You need to see how to access with st-link
or
usart1(pa9,pa10)or usart2(pa2,pa3) or usart6 (pa11,pa12)
But how i see on doc as i described above you have access on pa11,pa12 (if those pins are 5v tolerant no need for ttl converter from 5v to 3.3v)so cut an usb (another part you do not use for pc) and use (an ttl converter from 5v to 3.3v)
Usb Green wire connect [to ttl converter (5v pin) another one of ttl converter (3.3v pin)] to pin 27 on mcu
Usb White wire connect [to ttl converter (5v pin) another one of ttl converter (3.3v pin)] to pin 35 on mcu
Usb black wire (
of Gnd pin on ttl converter gnd which you) connect on mcu gnd
Red wire 5v No need to use
To enter on Dfu mode you should need to connect pin boot0 and vcc(3.3v) with an jumper