The first step is to create the Users import in your DEV model. Note that in order to import Selective Access to multiple items in a hierarchy, you will need to repeat the same row for each list item that you want to provide access to. I have attached a sample file (see right hand side), and here is a screenshot:
After you have created the Users import, sync your changes to the PROD model.
3. Overwrite the Users.csv file from Import Data Sources
In the PROD model, you can access the Import Data Sources screen from within Actions settings. Once here, select New Source > Upload New File... and then provide the full Users.csv file. Note that you will not see a confirmation message at this point; however, this will overwrite the Users.csv file stored within the model.
4. Run the import in PROD
Once the file has been uploaded, you can run the Users import from the Imports tab within Actions settings.