This is a Heroku buildpack for using a legacy version of ffmpeg in your project.
It doesn't do anything else, so to actually compile your app you should use heroku-buildpack-multi to combine it with a real buildpack.
To use this buildpack, you should prepare .buildpacks file that contains this buildpack url and your real buildpack url.
$ ls
.buildpacks
...
$ cat .buildpacks
https://github.com/shunjikonishi/heroku-buildpack-ffmpeg
https://github.com/heroku/heroku-buildpack-play
$ heroku create --buildpack https://github.com/ddollar/heroku-buildpack-multi
$ git push heroku master
...
You can verify installing ffmpeg by following command.
$ heroku run "ffmpeg-legacy -version"
If you want to use your own ffmpeg binary, fork and rewrite following line.
https://github.com/shunjikonishi/heroku-buildpack-ffmpeg/blob/master/bin/compile#L10