SFTP / SSH user inside account

I need some help with the following. We build an application where files need to be delivered from the 'outside'. This will be an automatic process but the system that sends the files to our server can only do this by SFTP for security reasons. So I have 2 things that need to be done.
1) Enable SFTP
2) Give a new user restricted access to 1 folder for dropping files (Like FTP management within directadmin).

How do I do this?

1) are you using proftpd? if so, you can enable SFTP following these steps: https://help.directadmin.com/item.php?id=439
2) you can just use FTP management in DirectAdmin for this. If you enabled SFTP in proftpd, you will be able to connect via SFTP using the port you defined in proftpd.sftp.conf, with the user and to the location you configured in FTP management.