Which distro and which version of NTPd are you using?
If it's ntpd from ntp.org then there is a configuration file,
/etc/ntp.conf, which contains a list of servers. In that you can on the clients comment out the existing list and replace the list with your local NTP server. Then all machines pointing at that one server will at least be in sync. You can see more information in the manual page for ntp.conf
In order to turn one machine into an NTP server, you need to open port 123 in the machines's own firewall on the machine that will serve. You also need to tell ntpd to serve time, but many default configurations do that for you by default. If you want the NTP server and clients to authenticate when checking time, that will have to be set up.
Also, there are some peripherals you can buy to have your server be a stratum 1 time server.