For installing the Prusa MK3s Stock Klipper firmware please follow the steps below:
V1.2 is the most current stable but still testing and modifying
Your build environment will be for the Atmega2560
For installing the SKR Bear Klipper firmware please follow the steps below:
V0.2 is the most current stable but still testing and modifying
Your build environment will be for the LPC1768 or LPC1769 Make sure you check both bootloader for Smoothieboard and USB Serial
Klipper configuration refernce guide