Extract the downloaded archive somewhere convenient
In Lightroom, open the Plugin Manager
Click 'Add' and point it to the extracted filmlog.lrplugin folder
Export JSON file
Export your roll from the app as a JSON file and put it into the folder with your digitised frames. Make sure the file has the same name as the folder
In Lightroom select "Import FilmShots Metadata..." from "Library -> Plugin Extras" menu
Select frames to import
Simply select the FilmShots frame you want to import into each of the digitised frame and click 'OK'.
You can always go back and change the selection.
Write Exif into exported files
When you are ready to export your scans, simply activate the "Write Film Shots Data" action in the Export dialog.
It will automatically execute Exiftool for each exported photo and write the frame metadata as Exif into it