See here for instructions:

http://technet.microsoft.com/en-us/library/cc773263(v=ws.10).aspx

Some additional information

https://social.technet.microsoft.com/Forums/windowsserver/en-US/adb51d52-1dcc-436d-89f0-2faa5a3cf7c6/looking-for-a-quick-run-down-on-how-to-properly-setup-time-sync-for-server-2008-domain?forum=winservergen

See also

Configure client computer for time syncing