You can find installers for Before Dawn on github in the Releases section of the project. Installing on OSX or Windows should be as simple as downloading the appropriate file, and running it locally. Installing on Linux is a little more involved, and I will add documentation for that as soon as I can.
The first time you install it, you will need to run Before Dawn manually. The preferences window will open, and you can pick a screensaver and set whether you want Before Dawn to automatically load when your system boots, and you can tweak some other settings as well.