Use the onboard timers to generate an interrupt that toggles the green led on and off using only the register definitions provied by St and the CMSIS core.
Make sure your discovery board is plugged into your computer, and that you have the required tools installed. Then use your shell to clone the git repository and compile/flash your board.
git clone https://github.com/kolazojar/blinky.git
cd blinky
make flash