This script is currently not published as a Google Drive add-on, so cannot be easily consumed for re-use. I have published the code here so that you can use it for your own scripts.
- Open a Google Document
- Go to Tools -> Script Editor
- Paste the contents of Code.gs into the editor
- Click Save
- Click Run
- I haven't figured out yet how to use it for all of your Google documents
- I think the script needs to be adapted for Google sheets (e.g. they have no body)