Use Arduino IDE. Install the STM Cube. Connect the programmer to the SWD pins, then just hit upload. No need for any bootloader, or anything. It’s a really straightforward process.
I created a user’s guide for the Mosquito, the LEPTON is using the same MCU, please read the programing section [2.5.5].
Add the clock config (simple copy/paste) to the beginning of your code. I don’t have it now, there was another thread as @runger said.
Cheers,
Valentine
https://community.simplefoc.com/t/mosquito-board-new-design/1621/25