Download or clone the whole firmware and navigate to the root folder. Once your dev env is setup, you'll be able to type `make %KEYBOARD%-default` to generate your .hex - you can then use the Teensy Loader to program your .hex file.
(Note: replace %KEYBOARD% with the name of your keyboard.)
Several version of keymap are available in advance but you are recommended to define your favorite layout yourself. To define your own keymap create a folder with the name of your keymap in the keymaps folder. Create a `readme.md` and a `keymap.c` file to complete your new keymap. When you are done your file tree should look like this: