Create a task similar to [[https://phabricator.wikimedia.org/T139724| the GAR example]]. This requires that you manually look at the metadata file and analyze it to see what can be done with it to comply with Commons best practices.
Try to integrate as much of the available metadata as possible.
Try to use the metadata as intelligently as possible. E.g. try to link where possible or use existing templates. Try to minimise the cases where metadata is "just" used as plain text in the description.
Make the format similar to other similar batch uploads
Be aware of any multilingual aspects (e.g. {{other_dates}} for complicated dates, {{technique}} {{city}} etc.)
Build in fallbacks or warnings (e.g. if the description field is missing then make one using other metadata or mark the file as one needing to be manually fixed (ideally before uploading)