I’m making the assumption that you have a working 12.04 installation that’s connected to the internet.

First, your going to add the freenx repository, then update. Followed by installing the freenx software.

sudo add-apt-repository ppa:freenx-team/ppa
sudo apt-get update
sudo apt-get install freenx-server

Now tht you have the Freenx server installed, you will want to configure it to use the ubuntu-2d session for any Freenx clients. The ubuntu-3d session doesn’t work with  Freenx and will give you an error message “Failed to load session “gnome-fallback”".

sudo echo -e "\n#Use unity 2d for client sessions\nCOMMAND_START_GNOME='gnome-session --session=ubuntu-2d'"|sudo tee -a /etc/nxserver/node.conf

All done, you should be able to connect using NoMachine’s free client.

References:
Installation of FreeNX Server on Ubuntu 12.04 Precise Pangolin
How to Install FreeNX Server on Ubuntu 12.04 Precise Pangolin