Flashing the bootloader from macOS

These steps assume you have a Blue, Red or Black Pill board, a USB-Serial interface with 3.3V broken out to a pin, and a relatively modern macOS. I've tested it with a Black Pill on Mojave 10.14.

The aim is to go from a completely new out-of-the-box board with the nothing but the built-in bootloader, to uploading sketches from the Arduino environment over USB.

1. Set the boot jumper on the Pill board to "Serial Programming"
2. Connect the USB-Serial interface's 3.3V pin to 3.3V on your Pill board, and ground to ground.
3. Connect the TX pin on the USB-Serial interface to PA10 on the board, and the RX pin to PA09.
4. Clone the Arduino_STM32 repository into ~/Documents/Arduino/hardware
5. Select the appropriate .bin file from here, and download it into your ~/Downloads directory (I have a Black Pill with the LED on PB12, so I'm using generic_boot20_pb12.bin; if you have a Blue Pill or Red Pill you want generic_boot20_pc13.bin)
6. cd ~/Documents/Arduino/hardware/Arduino_STM32/tools/macosx/stm32flash
7. ./stm32flash -v -w ~/Downloads/generic_boot20_pb12.bin /dev/cu.usbserial-DN0091ZE (the characters after usbserial- might be different on your system)
8. Unplug the Pill board from the USB-Serial interface, restore the boot jumper to its original position and power it up by connecting it to a USB port. Congratulations, you now have a bootloader!

Next, follow the instructions here to set up your Arduino environment to build and upload to your Pill board.