STM32 Flashing Procedure, Yes it's COMPLICATED.

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To me the most difficult part of the STM32 build is loading the software to the processor module. Here I've tried to lay out the process in clear steps. This seems to work for me.

Flashing procedure:

•  build and upload the OnStep sketch from the Arduino IDE. 
• Install the IDE and find or create a folder named "Arduino" in your document folder
• go to digitalWriteFast, copy the digitalWriteFast.h file and paste it in the  Documents/Arduino/libraries folder
• Open release-3.16 - this is the OnStep sketch, expand it in the Documents/Arduino folder. Rename folder "OnStep"
• Get STM32 master - Copy it and open file Explorer and install it in C:/Program Files (x86)/Arduino/Hardware.
  paste & unzip the file in Hardware. It should create the Arduino_STM32 folder.
• Open the IDE, select File : Open Documents/Arduino/OnStep/OnStep.ino
• Then select Tools/Board/Board manager and install Arduino SAM Boards (Cortex M3)
• One more, install RTC by Makuna library:
  

  
  

Remember use the USB/serial adapter NOT the STM32's USB connector.

• With no power to the STM32 and the USB cable not connected to the PC yet.
• move slide switch to "Flash" 
• power the STM32 - switch on 
• plug USB cable in PC - Windows USB tone sounds 
•  You're sitting there at the PC with OnStep loaded in the Arduino IDE 
• Your parameters should be set to:  
  

• You've compiled the code.
• So select the --> (upload)
• Now wait for 
  Wrote address 0x0801d000 (100.00%) Done.
  Starting execution at address 0x08000000... done.

 * remove power from the STM32 - switch off
 * unplug USB line from PC
 * move slide switch to "Run"
 * restore power to the STM32 - switch on
 * plug USB cable back in if you want to control/monitor from a PC application