Works with Arduino IDEs version 1.8.x and later
Open the menu "File" > "Preferences" and add the following text to the box "Additional Boards Manager URLs":
After this change the Boards Manager list will include Fortebit boards (you can type "fortebit" in the search box to quickly find what you need).
Please see also https://github.com/stm32duino/wiki/wiki/Getting-Started for useful instructions.
Note: you don't need to install the STM32 package, but it must be in your index because its tools are necessary.