If instead of using the bootloader, you want to use a ST-Link you can instead plug your programmer into the 20 pin JTAG connector (J6 on the UFO board):
Then, the details of this step will depend on your programmer. If you're using an ST-Link programmer, open the ST-Link utility and connect to the device:
Load your `.hex` file and program the device with the Program and Verify button:
After this, you're ready to go - you can use the ChipWhisperer terminal to talk to your target. You might need to reset the target before you do anything else.