The latest stable release of Fountain Mode is available as an Emacs package via MELPA-stable.
In order to install packages via MELPA-stable, you’ll first need to add it to your package archives:
M-x customize-option RET package-archives RET
Insert an entry named melpa-stable with the URL
https://stable.melpa.org/packages/
You can then find the latest stable version of fountain-mode in
the list returned by:
M-x list-packages RET
If you like to live on the bleeding edge and prefer the latest but perhaps unstable version, do the above using MELPA. Unless you are already familiar with debugging Elisp code, it is strongly encouraged to use the stable version.