I'm so excited to share this buildpack with everyone because 1) of the amount of frustration, time, and energy to get ffmpegthumbnailer working on heroku and 2) it works. If it doesn't work for you, please let me know at john.k.lawlor@gmail.com.
Firstly, you'll need to use the "multi-buildpack" buildpack over at https://github.com/ddollar/heroku-buildpack-multi
That repo will show you how to set the BUILDPACK_URL config variable on heroku, and how to add additional buildpacks to your application. You'll want to add these two buildpacks to get ffmpegthumbnailer to work.
https://github.com/shunjikonishi/heroku-buildpack-ffmpeg
https://github.com/johnklawlor/heroku-buildpack-ffmpeg-thumbnailer
You'll also need to add your respective native language (at least I had to). If you're building a Ruby application (like myself), add this line to your .buildpacks file.
https://github.com/heroku/heroku-buildpack-ruby
If you're using a different native language, refer to Heroku's default buildpacks list in their reference guide here.
https://devcenter.heroku.com/articles/buildpacks#default-buildpacks