• Ask about client server IP, eg: 163.172.255.254
• Create folder for files:

  mkdir /files/163-172-255-254/

• Fix permissions:

  chmod nfs:nfs /files/163-172-255-254

• Add new entry to /etc/exports

  /files/163-172-255-254   163.172.255.254(rw,sync,no_subtree_check,all_squash,anonuid=1001,anongid=1001,insecure)

• Reload config

  service nfs-kernel-server reload

More info about creating and using NAS server: https://www.digitalocean.com/community/tutorials/how-to-set-up-an-nfs-mount-on-ubuntu-16-04