Download or clone the whole firmware and navigate to the keyboards/retro_refit folder. Once your dev env is setup, you'll be able to type `make` to generate your .hex - you can then use the Teensy Loader to program your .hex file.
Several version of keymap are available in advance but you are recommended to define your favorite layout yourself. To define your own keymap create file named `<name>.c` and see keymap document (you can find in top readme.md) and existent keymap files.