File System Service is basically a NFS server style service. The simplest example is to create your FSS in the same region and VCN as where the server is location that you would like to mount the filesystem on. Below is the screenshot of creating a filesystem with FSS.
Documentation: Create a FSS, Mounting a FSS
Once the above is completed click on this new service, then click on the export path and then on "Mount Commands" button which provides the commands to use on the target where you would like to mount this filesystem. Here is a screenshot of the mount commands:
On my ExaCS the nfs-utils was already installed and I used different names for the mount points. Here are the commands I used.
sudo su -
mount 10.0.0.49:/fss2exacs /scratch3
and then update your /etc/fstab so that it is mounted each time the server starts.
You will need to do this on each compute node of your ExaCS that you would like to access this filesystem.