apt-get install proftpd
nano /etc/shells
FILE-ADD:
/bin/false
FILE-END!

/etc/init.d/proftpd restart

adduser --home /var/www --shell /bin/false ftpupload
adduser ftpupload www-data
passwd ftpupload

nano /etc/proftpd/proftpd.conf
REPLACE: "DefaultRoot			~" FILE-END!

TLSEngine			on
TLSRSACertificateFile		/etc/letsencrypt/live/[your domain]/cert.pem
TLSRSACertificateKeyFile	/etc/letsencrypt/live/[your domain]/privkey.pem
TLSCACertificateFile		/etc/letsencrypt/live/[your domain]/chain.pem
PassivePorts			40000 50000

// optional:
chown -R ftpupload:www-data /var/www
chmod -R 777 /var/www