1) create an user and force root to be owner of it
cd /home
sudo mkdir john
sudo useradd -d /home/john -M -N -g users john
sudo passwd john
sudo chown root:root /home/john
sudo chmod 755 /home/john
2) Change the subsystem location on /etc/ssh/sshd_config
#Subsystem sftp /usr/lib/openssh/sftp-server
Subsystem sftp internal-sftp
and create a user section:
Match User john
ChrootDirectory /home/john
ForceCommand internal-sftp
AllowTCPForwarding no
X11Forwarding no