Pretty straightforward.
Install the necessary packages
apt-get --no-install-recommends install nfs-kernel-server
Create virtual root
mkdir /nfs # can be /srv, /export or whatever
Create a directory (or more) under virtual root directory
mkdir /nfs/home
Mount filesystems to be exported under virtual root directory
mount --bind /home /nfs/home
Make the mount points persistent. Append them to /etc/fstab.
/home /nfs/home none bind 0 0
Put the following in /etc/exports (assuming access is granted to the clients in the 192.0.2.0/24 IP network. Client access can also be specified as a single host using IP address or fully qualified domain name, or * character to grant access to all clients).
/nfs 192.0.2.0/24(insecure,rw,sync,no_subtree_check,crossmnt,fsid=0)
/nfs/home 192.0.2.0/24(insecure,rw,sync,no_subtree_check)
Configure the daemon. Edit /etc/conf.d/nfs
OPTS_RPC_NFSD="8 -N 2 -V 3 -V 4 -V 4.1"
Start NFS server (assuming OpenRC. See your init system’s documentation)
rc-service nfs-kernel-server start
Start NFS server at boot
rc-update add nfs-kernel-server default
See detailed (and more) instructions on Gentoo wiki.