Firestarter is an easy to install firewall application for Ubuntu which also has to the option to enable Internet Connection Sharing at the click of a mouse.
$ sudo apt-get install firestarter
Launch the application firestarter. The wizard will come up on the first run.
Enable Internet Connection Sharing in the wizard, and start the firewall. Now other computers connected to this machine should be able to access internet , provided they all have correct routes, dns servers and ip addresses setup