This repo is only compatible with flatpak-builder 0.9.3 and later due to some backwards-incompatible changes.

Just add this repo as a submodule:

git submodule add https://github.com/casept/flatpak-modules.git modules     
git submodule init         
git submodule update        

And add whichever module you need to your main .json under the modules key, for example:

"modules": [
  "modules/ffmpeg-3.2.4.json",    
  your-own-stuff-here

will add FFMPEG as a module.