Below is a document which describes the files and format required for a Google Local Inventory feed.

With a local inventory feed, if you have multiple store locations that share the same SKU values a separate store on your account will be required. (The inventory store usually has duplicate SKUs that pan across multiple retail locations so a separate store is required for this feed. See Local Products inventory feed image below).

Source: https://support.google.com/merchants/answer/3055932?hl=en&ref_topic=7288923 

Find your Store code:

Go to Google My Business > Manage locations > Store code

This link is to give you the setup instructions by Google from the start.
https://support.google.com/merchants/answer/3540291?hl=en&ref_topic=7023401&visit_id=636735651773253538-979133028&rd=1