Follow all steps accoring to these instructions:
STM32duino wiki - Boards Manager
or these...
Condenced version:
1 - Launch Arduino IDE. Click on "File" menu and then "Preferences".
- Add the following link to the "Additional Boards Managers URLs" field: https://github.com/stm32duino/BoardManagerFiles/raw/master/STM32/package_stm_index.json
2 - Click on "Tools" menu and then "Boards > Boards Manager"
- Select "Contributed" type.
- Select the "STM32 Cores" and click on install.
3 - Done.
Navigate to your corresponding STM32duino AppData folder ex.
C:\Users\[User_Name]\AppData\Local\Arduino15\packages\STM32\hardware\stm32\2017.11.24\
Clone or Download this project to your computer and copy the files to the STM32duino folder.
If you already have custom boards added just copy the necessary lines from board.txt.
If you have a fresh install of STM32duino just overwrite everything.