This time it is completely simple: All you need to do is nothing!
The package OpenSSH, which is installed by default in almost all Linux distributions and that is responsible for the fact, that you can log in to your banana via SSH, contains already a SFTP server. SFTP = Secure File Transfer Protocol, that means an encrypted FTP protocol.
That means, users who are set up on the Banana Pi can immediately connect the banana with a FTP client such as FileZilla. You just have to take care, that you specify "SFTP" as protokoll.