Make sure you have a newsletter ready prepared. You’ll need the name of it to carry out this import.
To automatically subscribe certain contacts to your newsletter, add a “subscription” column to your import file and use the name of your newsletter.
(! You will find the name of your newsletter in "Settings --> Subscriptions", and not in "Automation" !)
If your newsletter is called “Weekly_Newsletter”, add this value to your contacts’ subscription column, remembering that it is case-sensitive and making sure you include all spaces.