Install the IAR Embedded workbench, free trials are enough but that choice is up to you (https://www.iar.com/iar-embedded-workbench/#!?currentTab=free-trials)

Clone the repo from the github link at any location of your choice
E.g ( %USER% /Documents/IAR Embedded Workbench/Arduino_STM32)

Open the the IAR embedded Workbench IDE and go to File->Open->Workspace , Now browse to the directory where you saved the repository that contains the following folders [ Arduino_STM32, Libraries ,Utilities ]

From there Click on the Arduino_STM32 folder and select the file Arduino_STM32.eww

*.eww (prononunced as eww (ˈɪəuː) !!!! ) files are the workspace files for IAR EW but they make a lot of things very easy since they carry a ton of configuration

The IDE will load the project for you and the left pane will have a 'User' group (the folder like icon)

here the 'main.cpp' is the primary example code that tests almost all modules under development