Re: "You forget to mention in the guide you referenced that all the users set their DNS records to the server's IP. "
Ah but I did... in one my earlier replies I advised:
"I would make the OS X server to be the DHCP server for your network, you just configure server DHCP put in address range, router & DNS IP address of the server and the search domain and turn it on."
The relevant part is in red. If you had setup your OS X server to be the DHCP server for your LAN and configured the DHCP setting for DNS to be your OS X server (as I recommended) then your client machines on your LAN would automatically get this information and would automatically know that their DNS server should be your OS X server. Nothing manual to do.
This is actually a very fundamental principle / function of LAN network setup. If you are supporting / managing local networks and users I would highly recommend for you to also get this book.