Logging SFTP activity can be done (on most Linux systems) by editing /etc/ssh/sshd_config . Simply find:

Subsystem sftp /usr/libexec/openssh/sftp-server

And change to:

Subsystem sftp /usr/libexec/openssh/sftp-server -l INFO

Then restart the ssh daemon:

systemctl restart sshd

The info log level is just one of many, there are others, like VERBOSE, DEBUG etc, but usually INFO is a good compromise. To see the logs simply tail /var/log/messages:

tail -f /var/log/messages | grep /the/directory/i/care/about