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.
So it's actually in most cases completely unnecessary to put an extra FTP server. If you only want to copy some files from/to the BPi via FTP this solution is totally sufficient.
Who does not want to use an FTP client you can also access the files on your server with your file manager. Under Linux you only need to type
in the address bar. After that you are asked for the password and then you can directly access the files on your server.
Windows needs some extra software such as Swich or WinSCP .
But then it is also possible under Windows to involve your banana via the SFTP protocol as a network drive, edit files or copy files by drag'n'drop back and forth.