This page describes the process for settings things up with the Arduino_STM32 tools originally developed by Roger Clarke. These tools are now effectively unsupported.
The main alternative that is supported is the Arduino_Core_STM32 project. I don't know how much of what's below transfers to Arduino_Core_STM32.
In addition to a “Blue Pill,” you will need a 3.3V USB to serial converter.
Lots of pins on the STM32F103C8T6 are purported to be 5V tolerant. The main DC isn't, and I don't know if TX/RX are. So, play it safe and use a 3.3V serial converter. I used a SparkFun Serial Basic configured for 3.3V. Only the TX and RX pins and, optionally, power will be used, so you should be able to use a standard FTDI board as well.
Some serial converter boards put out 5V as VCC even when set to 3.3V mode. Applying 5V to the 3.3V main supply line of a Blue Pill will kill it. Carefully check your 3.3V serial converter board to see whether the VCC output is 3.3V or 5V.
There is a 5V input on the Blue Pill and a USB connector that can power the board from 5V. These go through a small 3.3V regulator.
GNDpins of the Blue Pill.
3.3pin of the Blue Pill if your serial converter board provides a 3.3V VCC output or the
5Vpin if your serial converter board only puts out 5V VCC.
Under the Tools menu, make the following selections:
To re-upload, push reset and upload.
To put the device into “use” mode, move the BOOT0 jumper back to the 0 position (i.e., to GND, away from the microcontroller IC).
The stm32duino.com domain went down some time ago, apparently because it exceeded the host's traffic limit. The forums are now back online, but not the wiki. You can use the Wayback Machine snapshot links to access unavailable pages.
https://github.com/stm32duino/BoardManagerFiles/raw/master/STM32/package_stm_index.json. However, this list is incompatible with the list at