Enabling SFTP is very easy and makes FTP protocol a lot more secure than traditional FTP because passwords are not sent in plain text which is susceptible to a lot of things such as packet sniffers, man-in-the-middle, etc.

Assuming VSFTPD has been installed (in the default location) edit the file /etc/vsftpd.conf and check there are security certificates installed and configured.

There are two entries which start with either:
dsa_cert_file
dsa_private_key_file
or
rsa_cert_file
rsa_private_key_file
Each specifying a file path to where the key and certificates are located.

The last step is to enable SFTP by adding the following line:
ssl_enable=YES
Save and exit the file and restart the FTP server sudo /etc/init.d/vsftpd restart