Open the Collections panel and create a new Collection
Import the CSV you exported from the original project
Map the fields from the CSV to the fields in your Collection, or create new fields on the fly. For reference and multi-reference fields, create a plain text field for now. Learn more about importing collections.
If you’ve exported more Collections, do the same to import them.
Create the reference and multi-reference fields in each of your collections. Then, manually update these fields with the information that’s mapped to the plain text field you created for these fields. Afterwards, you can delete the plain text fields from your collection.