First install NFS (if not already) with your package manager (apt / yum / ipkg / dnf etc)

(apt/dnf/yum/) install nfs-kernel-server

Create a mount point for the NFS shares, such as

mkdir /var/nfs

Change permissions on /var/nfs if required.

Create /etc/exports and add

/var/nfs 10.1.2.3(rw,sync,no_subtree_check)

Where 10.1.2.3 is the server that you're granting access to. If you want more than one server, list them one per line. If you want a remote to only be able to read, not write then change rw to ro.

Update the export tables with:

exportfs -a

Finally restart the NFS service:

systemctl restart nfs-kernel-server

And that's it, NFS is now enabled on your linux box and can be used.